QRP Pen circuit

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From: Michael Marmor (mmarmor@pluto.njcc.com)
Date: Sat Feb 11 1995 - 01:32:08 EST


Thanks to everyone who responded to my questions about the QRP Pen
circuit on page 52 of K4TWJ's book "How To Get Started In QRP."
This single device oscillator project, that only took an hour to
build, has lead to several days of learning about RF filters,
feedback, SWR and other stuff. There are two main issues people
have mailed me about; the output filter and the collector impedance.
 
Here is the schematic again so you can follow what I am talking about.
                              
                              +V
                  ____________|__
                  | | L1
                  R 10K (RFC) .01 T-50-2
                  | |-------||--------|--mmmmmm--|----Antenna
                  | c | |
             |----+---b (2N2222) | |
             | | e | |
             | R 4.7K | Cx C 1 C 2
             - | |---||-- | |
   Crystal ----- | | | | |
             - | R 330 | | |
             |----|-----------|-|------|----------|---------------Ground
                              |
                              Key
                              |
                              -V
The Filter:
The book gives component values to build the circuit for the 30, 40
and 80 meter bands by adjusting Cx, C1, C2 and L1. The values for
both 30 and 40 meters closely meet expected values for a 0.1 dB ripple
Chebyshev low-pass pi-filter according to charts in the Handbook. Yet
the 80 meter values seem wrong. I checked 1, 0.1, 0.01 and 0.001 dB
ripple 3-pole filters and none of them seem to match the book values.
Does the 80 meter version use a different type of filter than the other
bands or is it a mistake? Here are the values from the book and my
calculated inductance for L1 based on the number of turns on a T50-2
torriod.
 
Band C1, C2, Cx Turns on a T50-2 Calculated L
30 330pF 14 0.98uH
40 470pF 17 1.445uH
80 680pF 34 5.78uH
 
Assuming the filter should be designed for 1.15 times the operating
frequency (I got 1.15 from AK0B's previous qrp-l post); here
are the "correct" values for my 3.579 Mhz crystal frequency.
Design Frequency = 3.579 * 1.15 = 4.12 Mhz
                         
                           C1, C2 L1 Turns on T50-2
 1dB ripple (2.66 SWR) 1563pF 1.9uH 19.5
0.1dB ripple (1.36 SWR) 797pF 2.2uH 21
0.01dB ripple (1.10 SWR) 486pF 1.9uH 19.5
0.001dB ripple (1.03 SWR) 316pF 1.4uH 16.7
 
The filter design info is from the '94 ARRL Handbook p 2-52 Table 21.
The T50-2 is on p 35-4 Table 7 of the same book. T=(100)(sq root of L/50).
This formula for figuring the inductance per turn does not appear to
consider wire gauge. I used #22 wire to wind the torriod.
How do you pronounce Chebyshev anyway? :-)
 
Collector Impedance:
My questions about why the oscillator stopped at certain settings of
my transmatch (and into a 50 Ohm dummy load!) resulted in several mails
about collector impedance. A related issue was why my wattmeter
read 25mW output at high SWR and only 5mW output at lowest SWR.
I am still foggy about how these things are related. I did get
this much from previous posts:
 
Load on Collector = RL = (Vcc - Vce(SAT))^2 / 2 * Po
 
Assuming Vce(SAT) = .5v (from a previous post)
Po = .25 (from the text of the K4TWJ book)
Vcc = 9v
 
Then collector impedance is 144.5 Ohms and SWR would be 2.78
 
If you use Po = .005 as indicated by my wattmeter you get a collector
impedance of 7.2K Ohms which gives an SWR of 138.5!!!
 
Help me sort all this stuff out!! Why does my circuit have 50 times
less power output than what the book says it should have?
 
Whew!! Why is learning so complicated?
 
73 de AA2UJ - Michael Marmor - mmarmor@pluto.njcc.com


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