On Nov 20, 1:13*pm, flipper wrote:
On Fri, 19 Nov 2010 04:58:00 -0800 (PST), Patrick Turner
wrote:
snip,
and I had said...
, and bjts are renowned for being slow at RF, but
the Ft for MJE340 is well above where I wanted to use them I may have
tried BF469 which are video transistors with Ft at 60MHz which would
have been better. But I only had a lot of BF472 with are PNP, and
which require me to build the circuit "upside down" which I didn't
fancy doing.
You said.....
Sound a lot like a sand box 6ME8 beam deflection circuit.
I've been thinking of doing it in reverse, though. Self excite G1 with
carrier. Feed single ended AF to one deflection plate and use bias
(offset) on the other to control modulation depth.
I'm not sure what a 6ME8 beem deflection circuit looks like.
Well, it appears my observation of similarity was no 'accident' as,
according to this site, the Gilbert cell derives directly from the
beam deflection tubes, being essentially a SS 'emulation' of it.
http://www.analog-rf.com/mixer.shtml
According to that fellah the solid state versions suffered from what I
suggested, device mismatch, and what you mentioned, Vbe non linearity.- Hide quoted text -
I'd read the James Long site
http://www.analog-rf.com/mixer.shtml and
that bloke has a loud grumble about just about everything.
OK, maybe he's mainly right but its all history now.
But for a discrete part circuit without any NFB to keep the SS devices
linear, the gilbert cell I made up wasn't too bad a performer even
without the tweaks I may have done in addition to what I did do.
But I have a HP 606A, and it was worth getting that going.
I tested the 606A for the audio bandwidth capabilities. Seems as
though if modulation is set at 100%,
then for a 455kHz carrier signal, I got a flat and undistorted
envelope from about 50Hz to 2kHz. Above 2kHz the envelope suffered bad
distortion with the AF wave shape looking like bad slew distortion.
But with mod set at 50%, the AF bandwidth was 23Hz to 50kHz, -3dB, so
quite god enough for testing an AM radio AF pass band. Same for 550kHz
to 1.7MHz, So the Q of the tuned LC tank in the 606A was not too high
to cut sidebands. This makes evaluating the receiver BW easy, just
tune to the 606A, and record the detector AF bandwidth.
I have yet to evaluate the signal path of external modulation signals
to the 606A. The input resistance for external mod is 1k0, and perhaps
its my oscillator gene which can't give a 2.5Vrms undistorted sine
wave into only 1k0. It should be OK as I have a 6AR5 CF output from a
completely gutted then re-designed old Japanese AF oscillator someone
donated to me.
It does 20Hz to 220 kHz, flat, wien bridge with tuning gangs.
Its possible to avoid the whole existing 606A attenutator unit because
the box with tubes, coils and tuning caps has ONE signal lead out from
the secondary on the OPT. the other end of the secondary is grounded.
Its an SE output.
The lead out has a 50 ohm plug and socket, so an alternative lead out
to a different attenuator, maybe an external switch in a box can be
arranged to avoid the existing which wrecks the modulation % in a
queer manner afaiac if the fine tuning pot is used.
I don't have the schematic of the 606A attenuator. But its output
measures 50R DC resistance on each range and its constant for any
level, so basically its done right. It may have compensation caps in
there, and things I cannot estimate.
Patrick Turner.