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Message-ID: <5.1.0.14.0.20020219140019.00ac1d70@gemini.herts.ac.uk>
Date: Tue, 19 Feb 2002 15:07:25 +0000
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Subject: LF: Re: Vectorscope display
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Dear LF Group,

I had the usual crop of bouncing e-mails, but I hope I have now sent the 
vectorscope circuit to all who have requested it. Thanks to G3YXM for 
putting it on his web pages - so hopefully it will also be available there 
shortly. Correction - I don't seem to be able to send it to 
sm6lkm.jbeab@swipnet.se, Johan, so perhaps you can get it from Dave's site 
later, or is there something else I can do?

Re: Andy's comment:
...how about
the next version, a direct conversion receiver generating quadrature signals
at 137kHz then having masses of low frequency gain before the X/Y plot ?

A low noise, high gain version of the circuit would certainly be possible, 
but there are a number of problems to be overcome here:

The signal spectrum at the mixer output extends to DC, so any DC offsets 
will be amplified as well as the wanted signals. This would saturate the 
output unless offsets could be kept down to a few uV. Even then, the 
remaining offset would effectively be an unwanted 0Hz "carrier" in the 
demodulated output spectrum. A related problem is that the noise figure of 
amplifers, etc. increases drastically at very low frequencies due to 
flicker noise. Both these problems could be avoided by having AC coupling, 
but this would put a hole in the received signal spectrum around the LO 
frequency.

Also, if we low pass filter the I and Q channels in order to define the 
bandwidth, any mismatches between the filters in the two channels would 
result in errors in the phase quadrature between the 2 channels, which 
would be a problem if good image rejection between the sidebands were 
required in subsequent demodulation of the signal.

I think it would probably be possible to achieve a useable result for a 
dedicated 136kHz RX - a fairly simple bandpass filter before the mixers 
could restrict the bandwidth to a few kHz, and some pre-mixer gain could be 
applied, reducing the post mixer gain required and reducing the effects of 
offsets.

Cheers, Jim Moritz
73 de M0BMU