Nachtfee New Thoughts


Page initiated on 23 July 2013

Status: 10 August 2013


Contribution 2 + 3


Although, the Nachtfee project seemingly has come to a semi conclusion, there always bore thoughts in my mind - that there still are open queries which might be in some way solvable. As long as we have not yet found other relevant technical documents - we have to acknowledge: every attempt to answer some of the open queries is having a great deal of speculation.


Please remember first:


Shown is the Nachtfee LB 2 ground control screen

The due North transponder signal is the original Nachtfee 'order' or command signal which has passed the way upwards to the FuG 25a IFF transponder and after reception it is retransmitted towards the Nachtfee ground station.

This signal has passed through space twice, and when we neglect the transient time through the electronic circuitries distance is to be multiplied by the factor 2. Like in radar technique this time-delay is directly proportional to distance.

In the Nachtfee console, the deflection time-base of the LB 2 and the dual-trace CRT HRP2 .. is being delayed such that it will correlate with the original 'order' vector setting. This time delay is being countered by means of the so-called 'Range offset' control setting.


In this case 'Range offset' being adjusted at 140 km

In this case the upper blip or Nachtfee signal equals the 'order' or Command vector at which the Nachtfee command pointer is actually facing at. In our case pointing at due North (please see further down).

By this means it has become possible to keep control of the entire 'order' link up- and downwards.

This 'Range offset' control does not influence the Nachtfee signal (in the domain of time) at all. It only brings the upward and downward signal in line at the control CRTs (distance compensation).

The second signal blip is the TB signal send towards the controlling ground system. It have been proven in previous pages that there is no way around that in some way or another the ground station should be informed what the actual signal phase of the equally running aircraft-time-base is.

The latter for two reasons:

The to be bridged distance, which changes all the time, is also having an impact on the actual signal phase arriving at the aircraft display. Nachtfee is therefore provided with a curious control called 'Phase'.  Its setting can not be monitored what so ever directly on the two Nachtfee control screens (LB 2 + HRP2/100/1,5A).


Please click on this drawing as to open it in PDF

The 'Phase' control 'A' is changing the quartz controlled signal phase just before the mutual signal junction (where the red line becomes blue) which goes upwards constituting the 'order phase' manipulated as to provide a to be given command (orders meant for aircraft guidance); in the other direction via the 'Range offset' (control C). Changing the time-base-signal-phase such that the transmitted 'order vector' (at the 'order' compass) equals the painted 'order' at the controlling LB 2 screen.


The upper picture shows that the TB reference is pointing at, say, 45 . In our simulated aircraft system, the entire circuitry delay is being represented by this value; considering that 500 Hz equals 2 ms. Hence: 2000 s : 8 = 250 s.

This figure is far higher than one may expect from the wartime observations, where, according to Fritz Trenkle, the IFF signal (EGON) blip was displayed which lags 400 meters further up the distance scale (10 km target distance provided on the EGON range-scale a distance of 10.400 m). This would have implied, that when the Freya/EGON range reading was compensated for its own system delays, that we have to count roughly with a 1.3 s additional signal delay.

Comparing our actual system delay and what was practiced in German wartime days, it might be worth to undertake some additional experiments.


Do we have an idea where this exceptional delay figure might originate from? Yes, we do.

It could be caused by accumulating delays in the transistor pulse forming stages. As to provide sharp pulse edges the transistors being operated in a saturated mode. It is well known that this is causing considerable time delays. But, I want to keep the circuitries simple but operating effectively. It might prove, however, that these two parameters do inflict.

My thoughts go into the direction of designing a circuitry based on a kind of wartime technology; operating valve controlled stages.


My thoughts about the principle of the new circuit concept

Each signal chain has to be fit with two valve stages - as each one reverses the signal phase (turning it 180) and we want to keep the output phase in concert to the signal input.

It would be ideal when it can be managed that both - the TB control pulse and the to be controlled Nachtfee 'order' signal would be pointing at due North.


Would this not be causing ambiguities? Not necessarily!

Because the Nachtfee 'order' signal is being kept by means of the Freya-Polwender switch in a due South position - during the time the Nachtfee guiding system is being kept in a 'waiting for a new order' mode. Please remember, that the Freya-Polwender mode is only interchanging the two output wires of the Nachtfee data output. Only in a coherent system the signal blip will change 180 in signal phase. Thus, North jumps to South v.v. One would see at the LB 2 control screen two opposite signal blips. For the aircrew a due south blip indicating that no action is currently valid. Only after the Freya-Polwender switch is being set at normal, and the 'order' signal blip has jumped to due North a new 'order' or command is due to come.

It could, however, causing confusion when both 'to be controlled signals' overlap one another (both pointing more or less onto the same vector). 


An instant thought: what would happen when the TB pulse is being generated as to point at due South?

This might causing confusion, as the TB pulse might well interfere with the important 'order' or command: Pauke! Unless, a TB-pulse-blanking in the (simulated) aircraft system is being provided.

I have to digest the new implications first, before giving further notice.




For those interested in the full explanation please consider Nachtfee in the domain of time


The basic Nachtfee console signals

Please click on this drawing as to open it in PDF

The red signal path is derived from the quartz stage, after division its actual signal phase is controlled by the variometer designated 'Phase'.

After the R 4 output transformer the blue signal path splits into two different directions.  The yellow one are the returning, to be controlled, signals originating from the airborne system.


The upper blue signal path being fed onto the 'order' or command variometer, which is mechanically coupled onto he big compass like 'order' or command scale

The actual 'order' pointer is the small coaxial one.

The 'Range offset' being set at just  over 280 km.

The LB 2 control screen is the top CRT.

The 'Phase' control down on the left-hand side has no scale.

Those familiar with the behaviour of coherent signals, will not wonder that the 'Phase' control varionmeter in the red circuit line, does not change the mutual phase coherence between the Nachtfee output signal and the controlling time-base-signal. Explaining it briefly: when the 'Phase' variometer changes the actual phase of the signal just where the blue line splits, the changing value will be equal for both signal paths; therefore not influencing their mutual interaction. When the 'order' or command variometer is being set differently the mutual phase difference still is not being influenced by whatever the setting of the 'Phase' variometer control is. For example, when phase control A (next drawing) is causing a signal delay of, say, 10 degrees, this value will be equal - for both the upper and lower blue signal paths.



Viewing it in a different way

Please click on this drawing as to open it in PDF



Shown is the entire Nachtfee signal flow

Please click on this drawing as to open it in PDF


Digesting the implications of this drawing, one might ask himself does the new thoughts making sense?

One point to be noticed firstly, is that the transistor circuitries do inflict with the received  'order' or command blip via Z-modulation channel; being made visible at the simulated aircraft screen (HP scope). Considering the previous sentence, the to be controlled Nachtfee 'order' signal returning at the Nachtfee ground control LB 2 screen, never passed through the simulated aircraft transistor circuitry. It straight arrived at the antenna and passed through the IFF transponder electrically with a delay of about 1.3 s.

The only circuit which can cause a delay is the simulated ground HF modulator. This one relies on transistors as normal digital HF generators (like our R&S SMS) do not allow pulse modulated signals. Our modulator circuit might causing signal delays. However, these being compensated for by means of the 'overall Range offset control' (designated control C in the second previous drawing); interacting only with the controlling time-base.

The TB pulse being delayed only a single time in the simulated aircraft circuitry; as is the Nachtfee 'order' signal fed onto the simulated aircraft display (both flowing into opposite directions, the electrical circuits are equal). Hence, causing symmetry, in the delayed signal paths.


As to facilitate a better understanding of what our virtual symmetry is about:


The dotted central line marks (separates) the two signal paths - one upwards and one downwards, which both constitute equal signal trajectories, hence, data flow being symmetrical

Please click on this drawing as to open it in PDF



Symmetry is the stipulation of the Nachtfee controlling system

When we do not obey upon this system principle the Nachtfee system could never have been controlled.

  The crux is: that the system delay into the direction of the aircraft screen is similar to the delay time of the TB signal towards the IFF transmitter chain. Asymmetry exist in the fact, that the Nachtfee ground signal passes through both the receiver and the transmitter circuit; whereas the TB pulse only passes through the transmitter. This would cause, say, of 1.3 s or 0.65 s. Viewing the fact that the velocity in free space of an electromagnetic wave is 300 m/s, this signal delay can be neglected. Also bearing in mind, that quite usually an aircraft moves with a speed of 100 125 m/s, this asymmetry value falls within the operational uncertainties.



It might therefore making sense investigating whether the implementation of valve technology versus transistor techniques might change the overall behaviour of our simulated aircraft display setup.

Especially in respect to system delays.

Likely being indicated by a different adjustment of the painted TB pulse vector at the controlling LB 2 CRT screen.

We have noticed before, that the TB vector is a constant (fixed) system parameter.


The new experiments should be accomplished in a manner that it is possible to switch quickly between the two system techniques, as to judge (comparing) both behaviours.

Please notice also the different explanation: Explaining it a bit differently




On 7 August 2013


New Reflection




We have so far explained what might be done as to minimise the transient time caused by the saturated transistor circuitries; incorporated in the simulated aircraft display system.


It might, however, make sense to measure the actual time delays in the currently operated pulse-forming systems.


This block diagram section from the simulated aircraft transistorised interfaces is showing how this might be accomplished

Please click on this drawing as to open it in PDF

Measuring the transient time of the TB pulse-forming section might tell us what we would like to know. Principally both channels are technically equal. The TB-deflection signal is a sinusoid; the signal to be fed onto the transistor circuit is being 'first order' differentiated firstly (after some kind of distortion). This latter is necessary as the IFF system (FuG25a) handles short duration pulses of a few s only. Because of this fact, the Nachtfee ground sinusoid (data) signal has also to be differentiated before it can be handled by the Freya/EGON upwards signal-transmission path.



One aspect is still wondering me, where comes the TB phase-shifted spot from?

As it should be only a few micro-seconds after due North; please neglect the small spots.

We are talking about the bright spot at, say, 4 minutes passed the hour.

Please consider also the previous drawing. The bright spot passed through both the TB circuit first then followed by the Z pulse-forming circuitry (it is also being fed onto the IFF system (@ pin 9) as to be transferred downwards to the Nachtfee ground console for control purposes). However, these two circuits together causing about 24 degree phase shift (including the transient time inside the oscilloscope). One parameter which is difficult to calculate with is the amount of actual transient delay inside the Z-channel and accompanied video processing of the HP scope*. The Z-channel is being fed by the Nachtfee data- as well as EGON signal (@ pin 9). The EGON signal is for us not significant and is photographically visible as a dash/dotted line, owing to the relatively long camera exposure time. I would, nonetheless, like to deal all the time including the simulated EGON signal component- as this was what occurred during wartime operations too. We have already proved, that its existence is in no way inflicting with our experiments.    *Though notice, that the scope time-base is being switched off, and measuring 'delay time' is not possible.


Please remember also:

The painted circle constitutes a Lissajous figure. The horizontal channel being directly supplied from the PM 5193 synthesiser sinusoid output. As to obey to the projection of a circle the second channel, in our case being the vertical signal chain, is to be fed by means of an additional 90 phase shifter. In our case simply constituted by means of two equal R/C networks. After some amplitude matching a perfect circle is being painted at the CRT screen. The TB signal is picked-up just where the 90 phase shifted signal enters the vertical scope channel (notice the just visible white/blue wire).

That the TB spot is visible is only due to the fact that the interrupting time-window (NE 555 gating) is being by-passed (gating mode being switched-off).

Does it make sense to investigate whether picking up the TB pulse before the 90 phase shifter? I forgot whether this have been investigated already. This technique was adopted before all circuitries have been implemented in the system. It certainly will have some effect, but this might well going into the 'Range- and Phase offsetting'. It might even causing that the TB reference pulse painted at the LB 2 CRT being rotated over 90. In my understanding not very helpful. My guess is, that we still have to investigate whether diminishing the signal transient in the transistor stages is helpful. Even if it is proving being not, this experiment is likely bringing us nearer to the wartime technical circumstances. What, however, might cause is that the TB reference pulse is no longer in line with the Nachtfee time-base system. The circular deflection of the LB  2 CRT is also being caused by a Lissajous, in this case being created magnetically. Optimally, both phase pointers, after distance and/or 'Phase' compensation, should point into the same direction constantly (behaving like two quasi-synchronous clocks). 

My credo: a negative result is also a result!



On 10 August 2013




New thoughts came up


How can we build pulse-forming interfaces using valves in a simple way?

For it two valve types are to be considered RV 12P2000s or using LV1s instead.

The Nachtfee console uses an EF 14; which type is rare and rather expensive and difficult to obtain nowadays especially when these should be mint. I therefore opt for using LV1s.

According H.-T. Schmidt's webpage the main data are:


Page one of the genuine LV 1 Telefunken datasheet


The RV 12P2000 is having a quite low mutual conductance 1.5 versus 10 mA/V for the LV 1 which we therefore would like to adopt for this experiment.


Let us follow what came up in my mind first:

You can click at both lower schematics as to open them in PDF


The genuine TB sine wave is derived from one of the Lissajous oscilloscope channels; before this particular signal can be used as a reference signal it should be after some kind of distortion 'first order' differentiated. This type of circuitry is providing a signal from just the sine wave section where it passes through 0 or 180 degrees. We would like to operate just where a new cycle starts from 0 onwards. Such a circuit - when correctly dimensioned provides short lasting pulses. 

Let us now first consider how the new circuit concept hypothetically works



Visible is that we deal with two dc coupled amplifier stages

The next schematic below is a tinny bit different. As coupling device between valve 1 and 2 we implement a neon indicator lamp instead. The advantage should be, that we have created an additional threshold; as the neon lamp may have an ignition voltage of 40 to 80 volt, I suppose (maybe an additional resistor should be implemented as to limit the g1-cathode brake-down voltage). It is, of course, possible to replace the neon device by means of a 'zener' diode, but such a device was not yet available in those wartime days, thus its application should be omitted. The rest of the circuit description obeys equally.




This provision certainly increases the threshold function of this modified circuit

We know already that the anode of valve 1 is having most of the 2000 s signal cycle (500 Hz) duration a high voltage level (H); only during a few micro seconds - just when the sine wave vector pointer goes through zero degrees (creating a few s lasting step-response) in the positive direction - R 1 will be conductive and a current flows (Ua goes to low level against ground).

Though, how do the signal levels and stages graphically interact? 


The symbols on the left-hand side correspond with the ones in the previous two schematics


Let us repeat from the beginning of the pulse-form sequence. The TB pulse after 'first-order' differentiation triggers the process. Thereafter the pulse returns to its low level sequence until the sine-wave signal crosses through 0 again. Let us, however, consider first that this is the real case. Which isn't entirely true, as the same circuit is also to be used for pulse-forming purposes as to supply the Z-scope-channel appropriately with signals originating from the IFF receiver section (@ pin 9). For optimal understanding how the circuit is interacting internally we imagine, for this occasion, that we deal with a TB like signal.


Status A:    The process starts when the TB input jumps from a low to a high signal level, thereafter the input signal level drops to a low again.

Status B:    When g1 of valve 1 (R 1) is surpassing its cut-off point in a positive sense, the valve starts immediately conducting and its anode voltage drops to a low level. After the TB pulse reaches its low level status again - the anode of valve 1 jumps to a high voltage level, as the valve being kept cut-off by the supplied negative grid bias.

Status C:    The situation equals B. As long as long as the TB pulse exists valve 2 (R 2) being kept cut-off due to its negative grid bias. After the TB pulse returned to its low level sequence the anode of valve 1 and thus g1 of valve 2 (R 2) becomes at a high signal level, causing valve 2 to conduct and its anode getting low. (Amplitudensieb)

Status D:    The valve is forced now in a conducting mode and its anode voltage will be at a low level; unless the TB pulse sequence starts again. 

We have got now a similar situation as is provided at grid one of valve 1. The only thing that occurs is that the signals being amplified and smoothed from lower level interference. The latter aspect is important when the same type of interface is being operated for handling the signals originating from the IFF receiver output (@ pin 9).

For reasons of simplicity, I have drawn a basic schematic only. The suppressor grids (g3) might be operated for blocking (blanking purposes) at will; like we use in conjunction with the (NE 555 gating). By the way, the negative grid bias can be derived from the floating valve filament supply (12.6 V ≈ 17 V dc). Which eases the supply provision.

In some respect worrying, is the fact that when pentodes being operated in a fully conducting mode (anode tending to a very low voltage against ground) that g2 will act as being an anode. However, I guess the LV 1 is robust enough. What might be experimented with - is, nevertheless, to wire the valves like behaving as a triode (anode + g2 and g3 interconnected). A technique rather commonly used in German wartime days. I must, nonetheless, admit that the pulse-forming stage inside the Nachtfee console (EF 14) is operated like a pentode without problems, though, not in a dc coupled manner.


My first aim is to build a single interface; and discovering experimentally whether its transient delay is far less than is obtained within the existing transistorised modules.


The purpose of this principle consideration is to have an idea where to start from when autumn time is due


Please consider next: how our circuit concept is responding


To be continued in due course


By: Arthur O. Bauer