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4hv.org :: Forums :: Electromagnetic Radiation
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Colpitts crystal oscillator driving small RF reference signal amp
Move Thread LAN_403
Carbon_Rod
Thu Aug 08 2013, 06:15AM Print
Carbon_Rod Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
 		I'm building a few cheap low power stable XTAL RF sources for some projects.
And ran into an ambiguity in the output calculation...

SGA7489 specifies 1 db minimum, but seems to be missing halve the notes :
Link2

C = Colpitts XTAL Q1 unbuffered signal: 50mV Peak-to-Peak
S = SGA7489 amp LF gain typical: 21 db @115mA
Vsupply = 5.0 V DC
VRFout = (C * 2^(S / (6.0 db_per_V_double))) * (1000 mV/V) = 0.565 V Peak-to-Peak
Zline = 50 ohm

Any insights would be welcome,
Rod
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Steve Conner
Thu Aug 08 2013, 08:57AM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
 		0.565Vpp across 50 ohms is 8mW, not half a watt.

Vrms = Vpp/(2*sqrt(2))

P=Vrms^2/R
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johnf
Thu Aug 08 2013, 09:01AM
johnf Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
 		carbon
you are jumping between voltage gain and power gain
and therefore potential difference and emf
gain at low feqs approx. 200 1dB compression point @ 150mW.
Try running your figures again and watch how much power you are applying to the crystal. Too much they do not like
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Carbon_Rod
Fri Aug 09 2013, 01:24AM
Carbon_Rod Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
 		Thank you both for the tips,
and Steve's answer appears consistent with the following results.

RFpeak_power_input = -22 dBm (50 mV Peak-to-Peak or 17mV rms)
RFamp_power_gain = 22.4 dB
RFamp_attenuation = 1 dB
RFpeak_power_out = RFpeak_power_input + RFamp_power_gain - RFamp_attenuation = -0.6 dBm
RFpower_out = 10^(RFpeak_power_out/10) * (Watt/1000mW) = 0.00087 Watts

@johnf
On the Colpitts section, a 35 gain follower (for example) would take the RF peak Amp input of 7 dBm to produce 0.436 Watts. There would also be more noise injected into the amps, but I've yet to test the design to see how much.

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