[Collins] 75A4 Roofing Filter ?
Dr. Gerald N. Johnson
geraldj at storm.weather.net
Sat Jan 24 12:08:22 EST 2009
On Fri, 2009-01-23 at 19:14 -0800, Adam Farson wrote:
> Hi Jerry,
>
> Many thanks for your comments. The nicest tube mixer I ever encountered was
> the 7360, which I used as a combined RX 1st mixer and TX balanced modulator
> in a transceiver which I designed and built in 1964.
Yes that tube had advantages, but not enough to keep it in production a
long time.
>
> Modern up-converting receivers generally have a bank of switched half-octave
> RF bandpass filters ahead of the 1st mixer. The better receivers switch
> these filters with relays or MMIC switches rather than diodes, and often
> incorporate a high-Q tuned preselector in the RF chain.
Sometimes they have the filters, sometimes not. And sometimes the relay
contacts aren't good for low level and go open with time. Sometimes
those relay circuits need biases added to clean the contacts. And
sometimes they don't allow for the second mixer to have greater head
room than the first and that leads to greater intermod within the
roofing filter pass band (e.g. closer than 20 KHz spacing).
>
> Cheers for now, 73,
> Adam VA7OJ/AB4OJ
>
There are many innovations, and some trips backwards in modern receiver
designs. There is much to do to handle the modern RF environment
including much attention to phase noise.
Last October I gave a presentation at Microwave Update 2008 on phase
noise. Yesterday, I posted my annotated PPT slides at
http://www.geraldj.networkiowa.com/papers There are two versions, one
power point and one pdf. K0CQ2008MUD.pdf and .ppt. It will probably show
up some day on the NTMS archive site.
Phase noise from local oscillators and interfering signals is a real
problem on HF and through microwaves. And there is more to learn that
I'm working setting up experiments to find out. Good phase noise is a
characteristic of the tube type Collins receivers with crystals for the
high frequency mix, and the PTO for the tuned mix. No synthesizers. I'll
probably use my 75S-3B in my phase noise tests this year.
I have been studying alternatives to the 6BF5 in S line receivers. I
need to organize my conclusions and make a web page of them soon.
--
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer
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