Hi Alan, This will be my first attempt at making a RF sweeper, are these similar to the old Wobulators for aligning receivers I.F stages or could they be used for this purpose.

Suri,

How much frequency chirp do you need?

This was designed for fairly narrow sweeps, you could change the sweep range relative to the center frequency by increasing the 3.3 pF coupling capacitor between the tank and the varactor.

BTW, spice might not be modelling the capacitance variation of the reverse-biased rectifier diode I am using instead of a real varactor.

Regards,

Alan

Hi Alan,

I have sent an email to alany@alanyates.com . But it got return. So I re-post in here. I would like to design a chirp signal and I have interested in your RF sweeper in website and simulated by LTspice. However, I did not get the RF output sweeping like chirp signal. I cannot attach the simulation file here (LTspice) here. I wonder that your RF sweeper is chirp signal or not?. Is that possible you could recommend me how to design a chirp signal from your circuit?

I am appreciated your help!

Thanks,

I'm back.

What is the max deviation, lets say at 7100khz, can I expect with your sweeper?

Deviation? Range of sweep?

TU Vern

Alex,

The BF245 should work fine... Otherwise try something like an MPF102 or J310. Just about any small signal N-JFET should work in a Hartley oscillator at low-HF.

Leave out the buffer and get the oscillator working first. Initially at a fixed frequency without the varactor tuning. It is possible that a very lossy varactor can prevent oscillation. The simple minimum-component Hartley almost ever fails to oscillate. You can cap-couple the top of the tank to the gate and DC bias it through a high-value resistor, some FETs don't like direct connection to the tank. Also measure the FET parameters Idss and Vpp to ensure the source resistor is appropriate for at least a few hundred microamps of bias current.

Regards,

Alan

Hi Alan,

I build 555 circuit and RF circuit using BF245. It´s don't work. What's the best substitute for 2N5484 ?

thanks,

Alex PY4EU

Vern,

It isn't too critical and depends on the core size you are using for mechanical limits and ease of winding. Thicker wire is good for high Q tuned circuits to minimise the copper losses. Generally with toroids you pick the largest wire that will fit the desired number of turns comfortably through though the core. Proximity effect isn't as big a consideration as it can be for solenoidal coils as only the parts of turns near the centre of a (single layer) toroid are closely spaced. You generally can't get a Q over 250 anyway because of the core losses, but the volume you save and the shielding more than make up for that.

IIRC I used 0.25 mm wire for this particular hack. I'd suggest thicker, say 0.5 or 0.71 mm wire for the tank, especially if you are using a nice high-quality low drift core like a type 7/white. For VFOs I generally use a T68-7 core and dip the finished assembly in wax to stabilise it.

The transformer is less critical because it isn't high-Q or operating at high powers anyway. A pair of 0.2 mm twisted bifilar the wound onto an FT37-43 core would work nicely, especially for a 4:1 collector load transformer reflecting 200 ohms to the collector of a feedback amplifier.

Regards,

Alan

Alan: What is the wire size for L1 & T1/T2?

Note that the drawing shows a T1 & T2 and I assume they are the same.

Will appreciate a reply.

TU de Vern

TU for the info and comment.

I need a sweeper to cover a few Khz. I am trying to build the W3PM/GM4YRE package. I hope your sweeper does the job to help me align the mixer/xtal filter. All the mods you suggest will be tried if I find the sweeper is lacking as I go along. Not an RF guy here. At 80 yrs it will be a challenge. Tks again.

73 Vern W9HLY

Vern,

Its a European standard where the radix point is replaced with the multiplier. So 3p3 is 3.3 pico Farads or 3.3 * 10^-9 Farads. Similarly 120p is 120 pF. The unit is left off where the circuit diagram context makes it obvious which unit it is, Farad, Ohms, Henry, Volt, etc.

This sweeper isn't the best design to copy. The oscillator is OK, but the buffer amplifier has insufficient reverse isolation, especially into highly reactive loads like crystal filters outside the pass-band. You can pad-down mismatch problems to some degree, say with a - 6dB Pi pad, but then more output power would be desirable. It would be better to use a more loosely coupled JFET follower feeding a common-base isolation amplifier and then perhaps a power amplifier after that if you need a few 10s of dBm output.

Regards,

Alan

Want to build your circuit. However, I am not certain what your cap value notation is when you say 3p3 and equally so when you say 160p. I think I know but want to be certain.

Thank you,

Vern W9HLY