From: epacyna@auratek.UUCP (Edward Pacyna)
Newsgroups: rec.radio.amateur.misc,sci.electronics
Subject: More on contructing simple low cost hi performance receivers
Date: 12 Nov 92 16:29:37 GMT
Followup-To: poster
Organization: Aurora Technologies, Inc.; Cambridge, Mass.


There has been a fair amount of interest from my orginal posting on building
high performance, simple HF receivers, compact low cost receivers. Following
is additional information that has been requested.

The Micro 20M receiver was orginally published in Ham Radio Magazine, Feb. 1988.FAR Circuits made a PC board (1 1/2" X 3", ~$5). Contact info is FAR Circuits,
18N640 Field Court, Dundee, Ill 60118.    

The author for the Micro 20 published two transceivers (using this receiver).
1. NE 602 Based 20M QRP Transmitter, Ham Radio Jan. 1989
2. QRP 15 Transceiver, CQ Sept. 1990
Note: The MFJ line of QRP transceivers are basically the above transceivers.

The Micro 20 also has audio derived agc (see below). Its good for SSB, but due
to loop response of audio signals, its attack time is slow and tends to pop on
attack. For CW rigs, I don't include it and just use a voltage divider circuit
with a potentiometer, the wiper through a 10K series resistor supplys 5V (max)
to 12V (min) to the MC 1350 agc pin (5). 

For those who might want to SWL, the NE602 Product Det/BFO can be replaced with
a envelope detector (a diode).

  Band Pass---NE602 Mix.---Cohn Xtal---MC1350---NE602 Prod.Det.---LM386 AF
   Filter         |         Filter        |           |             Amp. 
                 VFO                      |          BFO             | 
                                          |                          |
                                          |_________AGC Amp._________|



Following is a varation. The IF amp is elimited and replaced with a AF preamp.
The advantages are:

1. Better dynamic range due to less signal at the Prod. Det. stage. NE 602's
   can get bashed and be careful when using them as a transmit mixer.   
2. Circuit also shows a different AF power amp. The MC34119 was designed for 
   low voltage circuits. It has less audio distortion and you shape its band
   pass with external R's & C's to be optimized for commucication audio. See
   Motorola data sheet.

  Band Pass---NE602 Mix---Cohn Xtal---NE602 Prod.Det.---NE5534---MC34119 AF
   Filter         |         Filter          |           Pre Amp     Amp.
                 VFO                       BFO

  Should be able to build a portable receiver ( 6V battery,shirt pocket size)
  with this design.

Either of the above circuits and easily be made into a simple switch selectable
multi-band unit by using another NE 602 to premix the VFO/Xtal oscillator to
generate a new stable VFO to feed the first mixer.

  Band Pass---NE602 Mix--- Same as one of above
                 |         _
            Band Pass Fil.  |
                 |          |     New
    VFO-----NE602 Pre Mix.  | Note: diode switch
                 |          | xtal's for multband
              Xtal Osc.    _|


The NE602 is a double balanced mixer with on board oscillator and buffer in a
8 pin dip package and requires only a handful of external parts. Its made by
Signetics and cost about $2. Its built for low power use ( i.e. 6V @ 2.5 ma).


73's 
Ed Pacyna W1AAZ   email: ed@auratek.com