EMRFD Message Archive 6540
Message Date From Subject 6540 2011-08-15 17:23:10 rcbuckiii Sweep frequency generator I built the Swept-Frequency Generator described by WI5W in the March 1994 QEX issue. I am having a problem with it and wonder if anyone else has built this unit.
I have one oscillator running and have the output set around 7.3 MHz. It is driving the first amplifier transistor with a 140 mv signal into the base. But I can't get anything out of the collector.
I didn't use the 2SC1424 transistors for Q1 and Q2. Instead I used MPSH10 transistors. I have tried changing Q1 to a PN5179 but I get the same results. I'm also using MPSH10 transistors for the oscillators.
The LM317 sets the collector voltage of the oscillators at 2.5 volts. I tried increasing the oscillator voltage to 5 volts but that didn't make much difference.
I'm at the point that I'm thinking of scrapping this design and trying a different one. I have the detector assembled but haven't tested it yet as I'm still trying to get the generator functioning.
Any ideas or suggestions are welcome.
6541 2011-08-15 18:51:12 Thomas S. Knutsen Re: Sweep frequency generator Hello.
I don't have the QEX cd around now, so this is off the top of my head.
You mention the collector voltage, but what is your Base voltage? 140mV p-p
is to little to drive an transistor in class C. I'm asuming the emitter is
There are several amplifiers in EMRFD that should be suitable.
73 de Thomas LA3PNA/AE5YS.
2011/8/16 rcbuckiii <firstname.lastname@example.org>
> I built the Swept-Frequency Generator described by WI5W in the March 1994
> QEX issue. I am having a problem with it and wonder if anyone else has built
> this unit.
> I have one oscillator running and have the output set around 7.3 MHz. It is
> driving the first amplifier transistor with a 140 mv signal into the base.
> But I can't get anything out of the collector.
> I didn't use the 2SC1424 transistors for Q1 and Q2. Instead I used MPSH10
> transistors. I have tried changing Q1 to a PN5179 but I get the same
> results. I'm also using MPSH10 transistors for the oscillators.
> The LM317 sets the collector voltage of the oscillators at 2.5 volts. I
> tried increasing the oscillator voltage to 5 volts but that didn't make much
> I'm at the point that I'm thinking of scrapping this design and trying a
> different one. I have the detector assembled but haven't tested it yet as
> I'm still trying to get the generator functioning.
> Any ideas or suggestions are welcome.
Please avoid sending me Word or PowerPoint attachments.
[Non-text portions of this message have been removed]
6542 2011-08-16 04:09:45 Donald Kelly Re: Sweep frequency generator Ray,
Thomas is correct that there are suitable circuits in EMRFD.
That said, Randy, WI5W, lives in OKC. I don't believe he will mind if you
call him for help.
If you want I can give you his phone number. E-mail me.
I believe he built the sweep generator for a homebrew spectrum analyzer.
6543 2011-08-16 04:57:51 rcbuckiii Re: Sweep frequency generator The base voltage is .587 volts. The emitter is grounded through a series resistor combinati 6544 2011-08-16 05:07:04 rcbuckiii Re: Sweep frequency generator Don,
I sent him an email through the ARRL a week or so back but didn't receive a reply. Possibly he doesn't have email. I'll keep working
6548 2011-08-16 13:04:37 John Marshall Re: Sweep frequency generator Ray,
I don't have that QEX issue but the WI5W Swept-Frequency Generator was included in the "Filters and Projects" chapter of several ARRL Handbooks. I'm looking at the schematic on page 16.49 of the 2000 Handbook now.
The 2SC1424 transistors are used in a pair of cascaded broadband amplifiers with resistive shunt feedback and emitter degeneration as shown in section 2.7 of EMRFD. With 10 ohms degeneration and 240 ohms shunt feedback, the amps should show a gain of about 10 dB per stage. That's a voltage gain of about 3.2 per stage or 10 for the two stages, assuming a 50 ohm termination. Because of the heavy feedback, the transistor type isn't very critical. The MPSH10 has an FT of 650 MHz, not nearly as high as the 2SC1424, but it should work fine in this circuit at HF.
So, if not the transistors, then what? Any chance you have mis-identified the leads of the MPSH10? My datasheet shows it with the emitter in the middle, which is different than the 2N3904 shown for the Q7 oscillator. An ohmmeter check might help here.
The base voltage of 0.587 that you gave in another message sounds low to me, barely enough to start the transistor conducting. If the oscillator was running, the RF on the base might have "confused" that reading. I would expect to see at least 1 - 2V DC on the base. The emitter voltage will be 0.6V lower, and the emitter current will be emitter voltage divided by the sum of the two emitter resistors. Kinda guessing here but I think you want 5 - 10 mA emitter current, and I think you will need to reduce the two 10K resistor values quite a bit to make that happen. I'd start with 4.7K and see if that doesn't get the emitter current up to where the collector shows some action.
I wouldn't give up
6553 2011-08-16 19:12:25 Ray Re: Sweep frequency generator John,
Thanks for the information.
It turns out my digital scope was lying to me or something is quirky with it. I pulled the 100 MHz analog scope off the shelf and looked at the output of the amplifier and see better results.
With the feedback resistors at 10K as in the article, I see 40 mV into 50 ohms. If I change the resistors to 3.3K ohms I see 300 mV into 50 ohms. These are peak readings on the scope and not peak-peak. My power meter doesn't completely agree as it shows -1.9 dBm for the 300 mV signal. But I guess they are somewhat in agreement.
You were correct about the RF voltage on the base confusing the meter reading. If I remove the drive from the oscillator, and with the 3.3K resistors in place, the B-E voltage is .747. The base to ground voltage is 1.769. With the 10K resistors in place, the B-E voltage is .617 and the base to ground voltage is 1.752.
Now the questi
6555 2011-08-16 22:00:18 Edard Guilford Re: Sweep frequency generator