[Premium-Rx] 100kHz converter outputs - more

[John Graham]

The Plessey PR2280 does indeed have an output that is peaked at 100kHz.

The RA1792 and 1650/6 just have lowpass filters, though looking more closely at the 1650/6 circuit, the 185kHz LPF is followed by an amp with a single tuned circuit that limits the bandwidth to approx. 50kHz [still 3x the bandwidth of the roofing filter]. 

I could understand a bit of headroom to avoid phase shifts at the edge of the passband, but this seems like serious overkill - the RA1792 LPF really is flat to 200kHz!


From: John Graham 
Sent: Monday, August 16, 2010 9:53 PM
To: Michael O'Beirne 
Cc: PREMIUM-RX 
Subject: Re: [Premium-Rx] Racal 1782, 178x series


Michael,

I agree that the wide bandwidth is on the face of it pointless; that's why I found it puzzling.

The fact remains that the 100kHz converter for the RA1792 is specified as flat to 200kHz at -0.5dB [p.9 of the manual Addendum] -- or at -1dB if you prefer to believe the Functional Block Diagram, and the converter for the Eddystone 1650/6 is stated to be 185kHz at -3dB [Manual section 4, p.4]. Yet both receivers have 16kHz roofing filters.

The 1773 and 1774 were just the 1771 and 1772 fitted with the MA1070 connector panel and mounted in a Creeth Field Transit Case (see Appendix 2 of the 1771/1772 Operator's Manual).

73s,

John G.


From: Michael O'Beirne 
Sent: Monday, August 16, 2010 9:19 PM
To: John Graham 
Cc: PREMIUM-RX 
Subject: Re: [Premium-Rx] Racal 1782, 178x series



Gents

All this talk of RA1781s and 1782s (which I too had never heard of anywhere) 
tempts me to ask the obvious question as to what else was in the RA177/8 
series.

The 1770 was the start with a VFO film scale neary identical to that on the 
RA117.  All I have of it is a brief pic and mention in SWM volume XXIX page 
757 showing a model.

The 1771 and 72 we know all about and ditto 1778/79.

What about the 1773/74/75/76 and 77 and the 1780 and 83?

When did manufacture finally cease?

Were these perhaps developments or specials?

The fact that the IF filter sets run to nearly 130 variants shows what a 
wide range of applications the sets must have been put to.

The 100kHz IF output was a requirement for all receivers for use by GCHQ, 
and the sideband at the IF output had to correspond to the sideband at the 
areial input, which is pretty indicative of the use.  I have a copy of 
Plessey's engineering submission to GCHQ for the PR2280 series containing 
the GCHQ technical requirement.  I don't recall any requirement for a vastly 
wide bandwidth.  Surely anything more than 13kHz would be pointless in view 
of the early filtering.

Pat has suggested that all manner of FFT gear was probably hung onto the IF 
output (and probably digital recording as well).

73s
Michael
G8MOB

----- Original Message ----- 
From: "John Graham" <john.graham02 at btinternet.com>
To: <premium-rx at mailman.qth.net>
Sent: Monday, August 16, 2010 11:09 AM
Subject: [Premium-Rx] Racal 1782, 178x series


I've been following the discussions about the RA1772 and the RA178x series 
with great interest.

One receiver that nobody seems to have mentioned is the RA1782. I am the 
proud possessor of two RA1782 receivers, and a couple of RA1781s that I 
bought for spares [they were very cheap, bought from a dealer who was moving 
to new premises and disposing of old stock!]

The RA1782 is the locally-controlled variant of the RA178x series [think 
RA1781 with an MA1072 front panel].
Most of the RA178x series were remotely controlled, and the RA1782 reflects 
that pedigree:
  a.. Instead of mechanically-switched MHz bands, tuning is continuous 
throughout the entire range, with "bandchanging" using the fastest of the 4 
tuning rates [625Hz, 2.5kHz, 20kHz or 5MHz per revolution]
  b.. Instead of the manually-tuned preselector, there is either a wideband 
front end or automatically-switched half-octave filters
  c.. Instead of rotary switches, there are illuminated push-buttons that 
latch logic circuits
  d.. consequently, the mode and bandwidth are independently selectable
All the filters are symmetrical; the RA1782s have 13kHz, 6kHz, 3kHz, 1.2kHz, 
750Hz and 100Hz filters; the two RA1781s have a 300Hz filter instead of the 
13kHz one. [i.e. all 4 receivers have the "incredibly rare" 6kHz filter!].
There is no SSB mode as such, but the "xtal" mode uses a 1.4MHz CIO, so all 
one would need to do is install asymmetrical filters.
I looked at obtaining crystals for BFO frequencies for +/-1.8kHz or 
whatever, but was put off by the cost [cheapest quote £37.50 each].

All the receivers have a converter to give a 100kHz IF output. This seems to 
be standard in those applications. [does anyone know why they all - RA1782, 
RA1792, Eddystone 1650/6, etc - have a 100kHz output that has a 185kHz 
bandwidth when the roofing filters are all around 16kHz?!]

I was able to get one receiver working without too much difficulty by 
referring to the RA1772 manuals, which cover most of the signal path, but 
the second receiver has faults in the control logic, for which I have no 
information, so that is on the back burner for the moment.

I'd be interested to hear from anyone else who has experience of these 
receivers, and if anyone can provide a circuit diagram for the front-panel 
logic and/or either/both of the RF input boards, that would be wonderful!

I also have RA6790 and RA1792 receivers, and I agree with the comments about 
their relative merits.

regards,

John G.
RS 86946