Message-ID: <001901c1b962$0e9eb460$6a01a8c0@cianciolo>
From: "PC" <paulc@snet.net>
To: rsgb_lf_group@blacksheep.org
References: <5.1.0.14.0.20020218184622.00aafa88@gemini.herts.ac.uk> <5.1.0.14.0.20020219140019.00ac1d70@gemini.herts.ac.uk>
Subject: LF: Re: Re: Vectorscope display
Date: Tue, 19 Feb 2002 11:25:34 -0500
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Hello Jim,

I did not receive an email with the circuit attached and was not able to
locate it.

Could I bother you to resend

Thank you

W1VLF
----- Original Message -----
From: "James Moritz" <j.r.moritz@herts.ac.uk>
To: <rsgb_lf_group@blacksheep.org>
Sent: Tuesday, February 19, 2002 10:07 AM
Subject: LF: Re: Vectorscope display


> 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
>
>
>