Dr Ronald Stiffler SEC technology

[itsu]

5 crystal test done with a 1mH (102) choke instead of the 1K resistor in the supply line (12.5V).

First set with the base resistor set at 100K:

nbr of x-tals    output voltage (Vpp)    input current (mA)
1                    32.2                            14.6
2                    33.7                            13
3                    34.2                            12   
4                    33.9                            12.5
5                    33.8                            12.1

Then with base resistor set at 60mA input current with 1 crystal = 23K:

nbr of x-tals    output voltage (Vpp)    input current (mA)
1                    39                               60
2                    41.4                            44
3                    42.8                            37.8
4                    38.9                            54
5                    39.5                            49.6


So there seems to be a turning point after the 3th crystal.

I also tried with even lower base resistance (>100mA input current), but things got unstable and
heated up the transistor, trimmer pot and even the crystals.
The leds on the last test where very very dimly on.

The SA did not show any change in main peak (13.56MHz) bandwidth, the Occupied BandWidth (OBW) measurement shows 66.666% for all 5 settings (1 to 5 crystals)

Itsu

[gyulasun]

Nick,
Well, at 15 V supply voltage the series resistor in the collector should be in the some hundred Ohms range at least
but as I wrote in Reply #589 its actual value depends strongly on the actual input current  the oscillator takes from
a higher supply voltage.
What is the input current now from 15 V and you find the transistor overheats with the series 68 Ohm?
Did you measure it and calculated the dissipation and confronted it with the 400 mW allowed ?? 
(this 400mW already makes it pretty warm and valid for 25 degree ambient room temp)
IT would have been enough for me if you simply described in a few words where you had placed that series resistor
back then, no need for showing where it is now. 
Thanks for the pictures 
Gyula

[gyulasun]

Hi Itsu,

Very interesting and quasi unexpected results for sure, thanks.

With your pF meter would you check the capacitance of the 5 paralleled xtals, please? 
(not in the circuit board but separately from any circuit, maybe using clips to keep the wire legs together,
and do not solder them)

You wrote:  "Could it not be that with more parallel crystals the Q goes up resulting in a higher output
(but smaller peak) and stronger dip in current at resonance?"

Well, the increase of the Q for the parallel xtals as a result may be possible, I can accept that. But I cannot see yet
how this higher Q has influence to invoke higher output? The xtals are not in the output side but between the 'input'
and output points of the active device, right? This way the high Q cannot increase output voltage (power)
while input current decreases.  If we placed a full LC tank in the collector and could increase its Q somehow,
then you were right with the stronger dip in (collector) current at resonance, I would agree.
I do not get what you meant with ("but smaller peak") ?   

I will return to this later, maybe tomorrow.

Thanks,
Gyula

[itsu]

Gyula,

with "but smaller peak" i mean smaller bandwidth, so higher Q, smaller bandwidth, higher output.

But i agree with what you say about "The xtals are not in the output side but".


I measured the 5 crystals with my LCR meter and the values are:

1 =   5pF
2 =   8pF
3 =  11pF
4 =  15pF
5 =  18pF 

So about 3pF parallel capacitance each.

Itsu

[TinselKoala]

Why not try crystals of _different_ frequencies in the parallel stack? The spectrum analyzer display should be interesting....