[ARC5] A Heterodyne Sweep Generator

[Dennis Monticelli]

If indeed some QRP radios are backing off on the bias current of the 602
for AGC purposes, that is not advised in my optinion.  Doing so will reduce
gain and help protect subsequent stages from overload, but it makes the
inputs even more susceptible to overload.  That is why forward AGC is
sometimes applied to bipolar transistor input stages or a voltage dependent
attenuator is used ahead of the mixer.  A simple JFET operating in its
"triode" region makes a decent front end attenuator.

Dennis AE6C


On Thu, Mar 13, 2014 at 10:18 AM, Fuqua, Bill L <wlfuqu00 at uky.edu> wrote:

>   In some QRP transceivers they feed the AGC back into the input pins.
> By reducing the input bias voltage the gain is reduced. I can't recall the
> exact radios
> that used this scheme but they derived the AGC voltage from the audio and
> applied
> it to the mixer which had a transformer directly connected to the input
> pins and a bypass to ground
> on one side. The AGC was applied to the bypassed side of the input
> transformer.
>   However, I would suggest that you set up the system for constant output
> and use a variable
> attenuator between the output and device to be tested along with the
> appropriate series resistance
> to match the input impedance of the device under test.
>
> 73
> Bill wa4lav
>
> ________________________________________
> From: arc5-bounces at mailman.qth.net [arc5-bounces at mailman.qth.net] on
> behalf of David Stinson [arc5 at ix.netcom.com]
> Sent: Thursday, March 13, 2014 11:20 AM
> To: ARC-5
> Subject: Re: [ARC5] A Heterodyne Sweep Generator
>
> > The NE602 has a useful amount of conversion gain - an advantage
> > in
> > receiver applications (which is what it was designed for) - and
> > often a
> > disadvantage in frequency-conversion applications,...
>
> Can the gain be throttled with negative feedback?
> I've used them but never looked into this.
> Back to yard work!
>
> 73 Dave AB5S
>
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>