[ARC5] A Heterodyne Sweep Generator

[Fuqua, Bill L]

    Actually, if you look at the schematic of the 602 the inputs go to a pair of transistors that control the emitter currents of two differential pair of transistors.
The gain of a differential amplifier with current sink in the emitter is proportional to the emitter current. This is how the set of differential amplifiers multiply.
The pair of differential amplifiers each with differential current sinks in their emitters create a doubly balanced modulator.
   You can make a simple balanced modulator using a single differential amplifier with a current sink in the emitter and send one signal to the differential inputs and
the other into the base of the current sink transistor. Or you can have a fixed emitter resistor instead of a current sink on the emitters and apply one signal differentially to the inputs and the other as a common mode signal to the inputs. 
    However, what I had described is not the preferred means of controlling the gain of the system due to the fact that it does reduce its dynamic range. 
Another thing. We get caught up with the use of specialized ICs or modules these days when there were simple discrete solutions used in the past. As an example,
do you really need a balanced mixer?  A simple bipolar transistor works very well as long as the local oscillator drive is several time that of the input signal and you are not concerned about oscillator feedthru. 
A good example of a simple system is in one of the ARRL SSB handbooks. I don't have on in hand but it was a simple SSB filter type exciter using a single transistor Mixer followed by a crystal filter.
A  local vxo drove the transistor as well as the output of a diode balanced modulator(DSB) source. 
  There are some good Motorola notes on single bipolar transistor mixers and how to calculate their gain. Since there are no interfering signals there are no concerns regarding 
intermodulation distortion or cross modulation distortion. There are only two signals. And both are from oscillators.  

73
Bill wa4lav

________________________________________
From: Dennis Monticelli [dennis.monticelli at gmail.com]
Sent: Thursday, March 13, 2014 1:39 PM
To: Fuqua, Bill L
Cc: David Stinson; ARC-5
Subject: Re: [ARC5] A Heterodyne Sweep Generator

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<mailto: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<mailto:arc5-bounces at mailman.qth.net> [arc5-bounces at mailman.qth.net<mailto:arc5-bounces at mailman.qth.net>] on behalf of David Stinson [arc5 at ix.netcom.com<mailto: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


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