[R-390] Crystal check

[Bob Camp]

Hi

The resonant frequency you “see” in a test set is a result of the crystal *and* the load capacitance of the test fixture. More or less: The capacitance of the fixture “tunes” the crystal. 

Next up: If you have net C on both the input and output side of the fixture and are looking for peak amplitude, that also will impact what you see. You have built a “pi” network and it will match up best at some very specific frequency. 

0.00015% is 1.5 ppm. When manufactured, these crystals likely had a tolerance of +/- 20 ppm at the reference temperature. 

Depending on what crystal you are looking at, the reference migh be temperature like 85C. The room temperature number would be (many) 10’s of ppm off from there. 

The temperature in the radio will be warmer than room temperature. With the ovens off, it will be cooler than 85 (or whatever) degrees. The same “many 10’s of ppm” gets into the act here. What you see in the radio will not be what you see on a bench at 25C. 

Further complicating this is that they got tested in some very specific military test sets. Just what load those test sets presented when calibrated “by the book” was very much a matter of great debate. The answer back in the day was large banks of “reference crystals” that got used to keep everything on the production line reporting the same numbers. Replacing those test sets was not at all easy … 

Quick ppm math: 1 ppm is 1 Hz at 1 MHz. If you have a 10 MHz crystal and are shooting for 1 ppm, you look for 10 Hz. Way way easier than playing with percent. 

Fun !!!

Bob

> On Jan 6, 2025, at 12:50 PM, jkharvie via R-390 <r-390 at mailman.qth.net> wrote:
> 
> Making progress with servicing my EAC receiver and I thought it might be of some value to know how the crystals in the deck have aged regarding frequency and amplitude performance.
> 
> I am not aware of any specific language regarding how far out of spec the crystals can be and the receiver still operating well but this is what I found.
> 
> Using a scope and digital signal generator I conducted an evaluation of each crystal with a quick assessment
> 
> Results as follows
> Procedure for Crystal frequency performance assessment
> Tools needed
> Digital Signal Generator (0-60Mhz, 2 channel, multi waveform, 0.01 Hz resolution)        
> Oscilloscope - Tektronix TAS 475 (100 MHz- 4 Channel)        
> 
> Procedure:        
> 1) With care remove crystals from receiver, note placement position and orientation. Label each crystal for convenience
> 2) Set signal generator to 0.4 volts, waveform to sine wave        
> 3) Hook scope up to crystal in series with signal generator        
> 4) Adjust scope for best waveform presentation
> 5) At the precise frequency of resonance, the measured signal amplitude will increase, thus adjust signal generator frequency for highest crystal resonance peak        
> 6) Record frequency of crystal when peak is measured        
> 7) Replace crystals into receiver
> 8) If error is higher than 0.00015% offset from design frequency consider replacement of that crystal9) Note the voltage, at resonance the peak should be clearly higher 
> 
> 
> Crystal Number)Labeled Frequency(kHz)_resonancefrequency (kHz)_"delta(kHz)P"_"delta(kHz)F"
> 1)9000.00_9,000.78_0.78P (pass)
> 2)9500.00_9501.22_1.22P 
> 3)10000.00_10001.2_1.20P    
> 4)10500.00_10501.52_1.52P    
> 5)11000.00_11001.72_1.72P 
> 6)11500.00_11501.21_1.21P  
> 7)12000.00_12001.82_1.82P  
> 8)12500.00_12501.44_1.44P  
> 9)13000.00_13001.74_1.74P  
> 10)14000.00_14001.96_1.96P    
> 11)14500.00_14501.82_1.82P 
> 12.1)15000.00_15021.83_X_21.83F (fail)
> 12.2)15000.00_15000.07_0.07P (Replacement unit)    
> 13)15500.00_15502.11_2.11P
> 14)16000.00_16002.15_2.15P  
> 15)17000.00_17001.94_1.94P  
> 
> Thoughts?
> AlsoHappy New Year to everybody!
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