Bob Henderson [EMAIL PROTECTED] made an utterance to the drakelist gang
--
I understand it's bad etiquette to respond to your own posts but this is
more of an addendum than a response, as such :-)
Before I go further, I should point out I have a powerful feeling that if I
knew what I was doing, I might possibly be on the verge of achieving
something useful! The reality however, is that I obviously don't know what
I'm doing and consequently, although I can see the trees clearly, it's hard
to discern the wood :-)
Unable to avoid tinkering, I have set about reducing the input to the 1st
mixer from the RF stage. Having decided to do this, I first took the step
of improving the DR of the RF stage by reducing the value of the screen feed
resistor from 18k to 6.8k increasing Vscr from around 55V to 95V. I then
set about reducing the stage gain. I seperated g1 of the RF stage from AGC1
and instead applied a variable bias voltage. I currently have this set
to -7.5V. With this arrangement, all sign of IMD3 products on 40/20m have
entirely disappeared.
I have adjusted the s-meter to compensate for reduced RF gain and everything
appears hunky dory, though I don't believe it really is, even for a moment.
Essentially, an S9+40dB signal at the input now yields about 250mV p.p. at
the o/p of the RF amp. IMD3 products appear to be 80dB down. With about
2.1V p.p pre-mix injection this is an injection ratio at the input of the
1st mixer of around 8:1.
A less enquiring mind might consider this a fix but I'm not convinced!
Have I achieved anything more than a front-end attenuator would achieve?
What should the pre-mix injection level really be? With strong BC signals
on 40m, what is an expected p.p o/p from the 6BA6 RF stage? If I could only
lay my hands on the appropriate addition of Soap, I'm sure these, among
other, questions would be answered :-)
I'm fumbling around in the dark here. Does anyone have a torch?
73
Bob, 5B4AGN
- Original Message -
From: Bob Henderson [EMAIL PROTECTED]
To: drake mailing list drakelist@www.zerobeat.net
Sent: Friday, August 12, 2005 8:37 AM
Subject: [drakelist] R4C IMD problem (was image problem)
Bob Henderson [EMAIL PROTECTED] made an utterance to the drakelist
gang
--
Oh no he's back!, I hear you say.
I finally got some time this morning to put the R4C back on the bench. I
have modified my test setup to use direct injection of signals via
attenuators. My signal generators are a TS570 and a TS870. Both are
producing 10W output into dummy loads from which signals are bled off
through 100k resistors to a common 50 ohm resistor. The output across
this
second 50 ohm resistor is fed to a 2 way coax switch allowing me to inject
the resulting two tone signal into either the R4C or a TS930. The TS570
was
running on 14100 kHz and the TS870 on 14170 kHz.
I injected the two-tone signal into the TS930 and it produced signals on
14100 14170 of around S9+50dB on the 930 s-meter. I checked for a third
order product on 2*14100 - 14170 = 14030 and it was audible but making no
discernible s-meter deflection. Somewhere around 90dB down. Next, I
injected the same two-tone signal into the R-4C. Signals at 14100 14170
indicated S9+45 on the R-4C and at 14030 indicated S9. Apparently, only
around 45dB down.
I then connected the 930 via *10 scope probe to the output of the 5645
roofing filter and tuned the 930 to that frequency. I tuned the R-4C to
14100 14170 and found 1st mixer output on 5645 at around S9+40. I then
tuned the R-4C to 14030 and the 930 s-meter indicated S8 or around 45dB
down.
At this point I believe I have reconfirmed the following:
1. The TS930 has adequate IMD3 performance to serve as a spectrum analyser
in this test.
2. My makeshift combiner/attenuator for the signals from the 570 870
introduces no discernable IMD of its own.
3. The R-4C IMD problem is caused somewhere in the 6BA6 RF stage and 6EJ7
mixer stage.
Next, I connected the output of the R-4C rf stage into the TS930 via my
*10
scope probe. I checked for signals on 14100 14170. These were
end-stopping the 930 s-meter so I added some attenuation to bring them
down
to S9+50. I checked 14030 and found the IMD was at S7 or around 65dB
down.
This is not good but not as bad as only 45dB down.
I increased the screen voltage on the RF amplifier 6BA6 and found I could
increase the gap between the two-tone signal strengths and the IMD to
better
than 80db. At this point I thought I was onto a winner. I reconnected my
*10 scope probe to the 5695 roofing filter output and checked for the
relative strength of the IMD to two-tone signals at that frequency.
Disappointingly the gap had narrowed to 40 dB.
My suspicions at this point are:
1. The IMD performance of the 6BA6 RF amp can be improved by an increase
in
