AKD Target Hf3 Communications Reciever Instructions Manual

Front end receiver modification for DRM: AKD Target Communications receiver. Model HF3. 
  
 
Summary. The receiver was modified and capable of receiving DRM, but performance was limited by 
the phase noise from the 1st
 Local Oscillator (LO).  
With the receiver on test, fed with a signal direct from a DRM modulator, the phase noise from the 1st
 
LO limited the SNR as reported by the receiver software to 21dB. If the 1st
 LO was replaced with the 
output from a Hewlett Packard frequency synthesiser,...

 
 
  
 
2nd
 Steps 
 
Although the receiver was able to decode robust mode signals at this point, the amplitude response 
was very poor. Working through the stages: the first IF filter (which was originally a 2pole filter) I had 
replaced by a 4 pole 45 MHz monolithic crystal filter, type 45G15B1. These did not match in well with 
the existing circuitry, and were giving an 8dB p/p ripple across the wanted frequency range. Adding 
additional buffer stages to provide the specified terminating impedances...

 
This gave an overall response RF in to 455kHz out at the input of the 12kHz converter board of: 
  
 
The response is significantly improved, but still shows some ripple and falls off a bit steeply at the 
high frequency end. 
With a bit of adjustment to the three 455kHz IF coils, the final response below was achieved: 
 
 
    
 
Response 10dB/div       Response 2dB/div 
Centre 455kHz, 5kHz/div  
 
 
 
3rd
 Steps 
With the receiver amplitude response virtually flat across the required +/- 5kHz...

 
HF3 1st
 LO phase noise  
 
HP synthesizer phase noise  
  

 
Software Receiver screenshot after all modifications completed.  
 
Receiver screenshot, 1st
 LO replaced by HP synthesizer  
 
Note, both look very similar on the spectrum display but with very different SNRs. 
20dB is just sufficient to decode the high data rate in use here.  

To Do: 
 
1. Further work is required to try and improve the 1st
 LO phase noise. 
 
2. The SSB ceramic filter needs to be re-integrated to restore the original functionality of the receiver. 
It should be relatively simple to add it back in-line after the added 455kHz circuitry, along with some 
switching circuitry to select either the AM or SSB filter as appropriate. 
 
 
Before: AKD Target Communications receiver. Model HF3  
internals: 
Red   1st IF (45MHz) filter 
Green SSB 455kHz filter 
Blue  AM...

  
After: Modified AKD Target Communications receiver.  
 
 
 
Purple  455kHz to 12kHz downconverter board. 
Green  additional 455kHz IF stages with wider (+/- 7.5kHz) ceramic filter. 
Blue     Replacement 1st
 IF filter  (4 pole crystal, +/- 7.5kHz). 
 
Modified receiver Block Diagram. Wider 
45MHz crystal filter +/- 7.5kHz 455kHz 
AM Filter Synth LO 45.03 
to 75MHz Crystal 
LO  
Demodulator AGC Additional 
ceramic filter 
+/-7.5kHz 455 to 12kHz 
converter board 12kHz IF out Audio  

 
 
 
 
 
 
 
 
  
 
 
 
 
 
 Software window, note good shape of 
received DRM signal (if you can see it). 
Using test encoder for ídeal’ input signal. Modified AOR 
7030 Target HF3 
under test Thales DRM encoder and synthesiser  

 
Checking the receiver IF response.  
 
Testing with DRM signal. The 455kHz IF is displayed on the spectrum analyser.