NE602

(1/1)

Paul:
The NE602 or NE612 can be configured as a VFO, so a simple qrp transceiver can be created with a sinle NE602 chip, acting as a mixer/oscillator on recieve and a vfo on transmit ! This can be done by bypassing input pin 1 with a 10 k resistor to ground and the oscillator frequency will appear at the output !

Claude Stewart:
Thanks, Paul. I saw your idea in the latest Sprat. A lot of us would be interested in knowing if you've tried it yet and the kind of results you've gotten.

Paul:
Hi all,
I have tried it , as a xtal oscillator & vfo, it works ok and a decent rf output appears at pins 4 & 5.
To actually put it in practice, will need some switching.
First, depending on the type of tuned input circuit on pins 1 and 2,  on receive (the chip used as a DC Rx)pin 1  needs to be grounded via a 10k resistor. So by merely inserting this resistor, the local oscillator output will appear on both 4 & 5 pins. I guess you can use pin 4 as vfo output to TX buffer & pa and pin 5 as the  demodulated received audio output to the audio stages, depending on the circuit built around  these output pins. Both controlled by this simple resistor on pin 1 !

Steve Taylor:
Hi

I came across this by accident just a few days ago. I have lifted the AGC circuit from the Elecraft K2 to use in my homebrew multiband transceiver. This is a fine circuit which uses an NE602 to produce a low frequency IF of around 150KHz (mixes the 4.915 IF with a 5.068 crystal) Elecraft use a 10K resistor to earth on one of the inputs to switch off the AGC, as Paul has noted this produces a hefty sine wave output from the chip at the crystal frequency. In my K2 this switches the AGC off nicely, but in my homebrew rig the AGC circuit sees this sin wave and applys a large amount of AGC to the IF amp, thus reducing the gain........i.e. not the desired effect. Just an observation.............I'll just leave the AGC switched on I think!!!

73 Steve G4EDG

Dale Hunt:
The NE602/SA612 series chips contain a double balanced
Gilbert cell mixer with pins 1 and 2 as the balanced
inputs.  The spec sheet says these pins should only be AC
coupled, though it won't hurt if they are connected
together for DC (for example, through a transformer).  
That is because the internal bias is applied to both pins.

If you change the DC level on one pin it unbalances the
mixer, and the oscillator signal will appear at the output.
Similarly, the input signal should appear at the output
even when the oscillator isn't running.  This is true of
any balanced mixer - many rigs apply a variable DC voltage
to the SSB balanced modulator to generate carrier for
CW operation, varying the voltage to adjust the output
level.

But if the pins are connected together through a coil
or transformer, the DC levels will be the same on each
and the mixer will still be balanced.  This is the way
that it is used for gain control.  If you are using
single-ended input you can just put an RF choke between
pins 1 and 2 and sinking current out of them should
reduce the gain without unbalancing the modulator.
Otherwise if there isn't a coil between them so they
are at the same DC level you can use a resistor from
each pin to a common point and use that to control the
gain.

Shifting the DC levels of the input pins may change the
operating point of the oscillator slightly.  This may
not be a problem with a crystal oscillator, but could
cause an undesired shift with a VFO.  In the latter case
it would be better to use a discrete transistor for the
VFO and feed it into pin 6.  In any case, you can tap
off the oscillator output from pin 7 and feed it to an
external buffer amplifier if you want to use the signal
in a different part of the circuit.

All of this (except the gain control idea) is covered
in the NXP (formerly Philips) datasheet and application
notes found here:

http://www.nxp.com

(You'll find most of the Applications Notes appear when
you search for SA602, but they apply to the NE602 and
SA612 series as well.)

Navigation

[0] Message Index