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Author Topic: FREQ COUNTER WITH OLD RECEIVERS  (Read 753 times)
VE6PG
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Posts: 1




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« on: April 25, 2005, 06:04:19 AM »

HI FROM TIM,VE6PG..I HAVE A FEW FREQ COUNTERS,USE THEM
FOR TRANSMIT WITH MY OLD TRANSMITTERS..HAVE TO BE SURE I'M ON FREQ..WHAT I WOULD LIKE TO DO IS,PUT THE COUNTERS IN USE FOR MY RECEIVERS..NC300,HQ100,BC348,SP600 ETC..ANY INPUT ON THIS?..PLEASE LET ME KNOW..73..TIM...SK..
  ve6pg@rac.ca
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WB2WIK
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Posts: 20574




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« Reply #1 on: April 25, 2005, 11:58:02 AM »

You need an algorithm to convert tunable oscillator frequency to actual received frequency.

For example, in a simple single-conversion receiver having a 455 kHz I.F., a bandswitched tunable oscillator might operate at 1.015 MHz through 30.455 MHz in order to receive 560 kHz through 30 MHz.  Your counter would read those frequencies, not the actual "received" (dial) frequencies, so you'd need to subtract 0.455 MHz from all frequencies displayed by the counter in order to determine received frequency.

(This is a very simplistic explanation, of a very simplistic receiver!)

With most receivers, it's more complex than this.  But in any case, there's no way to "directly" read the frequency a receiver is tuned to; the method is indirect, and you may need a buffer stage to isolate the frequency counter from the tunable oscillator, both to set the correct level for the counter and to isolate the counter from the oscillator.

WB2WIK/6
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KA5N
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« Reply #2 on: April 25, 2005, 01:05:39 PM »

There are several ways to accomplish using a counter to display a receiver's frequency.
1.  Build a mixer to selected the sum (or difference)of the local oscillator and the IF frequency and count the output of the mixer.  For example a 455 kHz fixed oscillator (crystal) and the local oscillator.  Since the local osc is usually higher in frequency you count the sum.
2.  Modify the counter so that you can present in an amount to effectively add in the IF frequency.  Normally this would be the one's complement of the amount you wish to add.  I modified a counter to read the operating frequency of my TEN TEC Arogonaut 509 this way worked great.
3.  For a multi-conversion receiver you can sum all the oscillators and add in the last IF.
I am sure that there are several more methods.  If you wish to use the same counter for several different receivers that may complicate matters.
Allen
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KA5N
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« Reply #3 on: April 25, 2005, 01:18:48 PM »

I mispelled preset as in preset in a number.
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W7DJM
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« Reply #4 on: April 25, 2005, 06:50:21 PM »

The short answer is,  you can't--at least not as accurately as modern radios, because the oscillators in the old gear are simply not that stable or accurate, and the center of the IF may be off somewhat.

You CAN however, get a nice little digital display that can be "adjusted" for different IF's as well as whether the L.O. is above or below the IF here:


http://www.aade.com/


There is one of these little units that can also be setup to display proper vhf/uhf freq on the readout when a RX is being used downstream of a converter.

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KA5N
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« Reply #5 on: April 26, 2005, 06:03:55 AM »

If the receiver oscillator is not accurate or stable then the display will indicate that the oscillator is not stable.  The accuracy of the display does depend on the accuracy of the setting of the IF freq.  I would say that the desire is to get a better idea of the operating frequency, not to have absolute accuracy of the display.  In many older receivers a display that was accurate within +/- 5 kHz would be a major improvement.
Allen
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WB6BYU
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Posts: 13152




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« Reply #6 on: April 28, 2005, 02:07:27 PM »

One advanteage with a preset counter is that you only
need to measure the frequency of one oscillator, and the
others in the receiver hopefully will be crystal
controlled for better stability.  Instead of programming
the preset for the nominal frequencies, tune in WWV and
tweak the preset until your counter reads 10.000 MHz.
This will take into account most of the possible quirks,
such as the IF not being aligned exactly to the nominal
frequency.
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