Increasing the range of varicaps, tips and tricks needed

Hi I have an mv209 varicap used in tuning a receiver to frequency. The available voltage on the receiver is 0-12v.

I am looking for any tips and tricks for increasing the range of the receiver (varicap tuning) without switching. This means reducing the minimum varicap capacitance, whereas at the same time keeping the max capacitange the same or even greater.

This is quite tricky and from a first look it cannot be done. What I am trying to do is to alter the capacitance/voltage curve without switching.

Maybe a relevant video can be seen here https://www.youtube.com/watch?v=kcdlJvPlJ3o but I do not have a coil, I have a ceramic resonator and a varicap shunted to ground to alter it's frequency.

Hi,

What frequency (-range)?

Opamp is too slow, I assume?

--> for HF: adding a phase shifted signal could reduce effective capacitance. But I′m not very experienced in HF.

Klaus

Yes it is for HF, 7MHz as a starting point.
An obvious solution is to use capacitors in series ans parallel in combination with a switch, to get several ranges. For example a series capacitor with the diode will reduce the total minimum capacitance but also the maximum.
But I am trying to see if something can be done without switching.

To increase the maximum capacitance, connect two or more in parallel, as they are used in non conducting mode and no current passes, you get the same effect as capacitors in parallel.

To decrease the minimum value, the only viable option is to increase the voltage across the varicap. I would consider ways to boost the 12V you have to hand.

To widen the range of capacitance, use both methods, put varicaps in parallel and also increase the voltage.

Brian.

I thought so...
I worry about the voltage ripple in voltage booster circuits, as this will directly affect the frequency stability. Not to mention that if this circuit operates close to the RX interference might occur, as this will probably include a lower frequency square wave oscillator.

Any proposed circuits will be appreciated. Discrete is better for me, if it is not too complex. Very low current is needed anyway.



I have already seen some ideas in this page https://en.wikipedia.org/wiki/Voltage_doubler. Can it be as simple as a Dickson charge-pump voltage-doubler driven by a hich frequency oscillator? Maybe a > 30MHz oscillaror is better to eliminate interference from harmonics on hf <30MHz. I have no idea if this would work on a sinewave or if it needs a square wave

The Dickson charge pump would be my choice. Ideally it would clock at relatively low frequency, up to a few 10s of KHz at most and ideally be driven with a square wave but it will still work with a more 'rounded' clock signal and that would help to reduce harmonics. For maximum output and minimum ripple, the clock level should be the same as the DC input voltage so you could possibly drive it from a multivibrator circuit or even an NE555 if you were careful to filter its supply pins. Note that the clock signal has to produce some of the output power but for varicaps that is a tiny amount anyway.

You should be regulating the varicap voltage for good stability. If you use an NE555 and two Schottky diodes BAT85 for example you should be able to get within about 1V of double the input voltage. With 12V in you could get about 23V out which you could Zener stabilize at say 20V. Note the PIV of the BAT85 is only 30V so be careful if you try for higher voltages using one!

Brian.

If I use it I think I will try germanium diodes, but in HV it should not matter too much. So It can't be driven with an HF RF signal, even if it is square? I wonder why? Just make the caps smaller and it should work.

In theory it will work at HF but in practice you will find it more difficult to generate a square(ish) wave as the frequency increases without using a more powerful source. Don't forget the load impedance on the clock is relatively low. If you look at how the pump works, one capacitor is charged from the input source when the clock is low and the the clock raises the ground end of that capacitor forcing the charge through to the second capacitor. That means (for a square wave clock) half the output current has to come from the clock signal itself.

You can use Ge diodes but they don't really have any advantage over a Schottky silicon type. In fact if you look at some diodes, the OA91 for example, Vf can be more than 3V at only 30mA. Thats 6 times more than a BAT85 at the same current.

Brian.

There was (is) a circuit on the internet that I cannot now find that had a very novel way of making a VCO that tuned over typically a 7:1 frequency range.

That is like a 50:1 continuously adjustable LC ratio !

This used the usual pair of varicap diodes nose to tail, but also it varied the inductance of a ferrite toroid as well, in the form of a saturable reactor.
The really clever part of this is that only a small section of the toroid was saturated, rather like a fake airgap. And apparently this is able to reduce the inductance without significantly lowering the Q.

I have a printout of the article around here somewhere too, but I cannot find that either right now. But this is a now famous circuit built and tested by many radio amateurs and it definitely works very well as a very wide tuned VCO with excellent tuning characteristics for HF and VHF.

I think I know which circuit you refer to, but I cannot find it either. But why such a complex circuit? an HF multivibrator with 2 transistors is all needed to supply almost square wave pulses nead the VCC amplitude. Frequency stability is not important at all.

As I recall it used an MC1648 oscillator which has an automatic oscillation level control, with a choice of either sine or square wave output.

If you are going to build a very wide ranging oscillator, the problem can often be that there is a huge amplitude variation from one end to the other, or it may drop out of oscillation altogether at some point if the LC ratios become extreme.

For many applications an almost constant output amplitude over such a wide frequency operating range can be very useful.

Not, it is not a wideband oscillator I am building but a receiver. The receiver uses a ceramic resonator and I am pulling it's frequency using a shunt varicap. The min capacitance of the varicap, determins the highest coverage frequency and the max capacitance, the lowest. The receiver is regenerative so no need to worry about constant oscillator level anyway.

This was the original award winning article that started the whole original idea off.

http://www.techlib.com/files/crvco.pdf

There have been other discussions on various Amateur Radio websites that show a much clearer schematic, and go into some of the practical details of building it.

Quite a few have been built, and the performance has proven to be very good.

The link to the design is here http://www.robkalmeijer.nl/techniek/...e61/index.html

What frequency and tuning range do you need Neazoi?

Brian.

Haha, we posted simultaneously. Mine was the original article, but yours is much better and the one I was really looking for.

Yes I know you are wanting to tune a receiver, but you may be able to adapt this idea or something similar to it.



Sorry, I missed this part.

You are really not going to be able pull that resonator very far. I built a 500 Khz BFO for an HF transceiver project using a ceramic resonator and a pair of varicaps.
I just had a look at my notes and about +/- 3 Khz was as far as I could make it go.

I bought ten resonators and tested all of them to find the one with the highest frequency so I could reach the upper limit.

Great minds think alike!

Worth mentioning that the MC1648 went obsolete many years ago and was functionally replaced by the MC12148.

I have some reservations about the original design which, although sounding plausible, makes me think the IC may not be working quite as described. I could be wrong but with the values shown I have a feeling the common base amplifier may actually be the oscillating stage with the IC working as a signal pump to injection lock it. Construction would be critical and the current through the reactor would make it relatively inefficient in power consumption.

Brian.

The saturable reactor needs up to something like 100mA if I remember, so its certainly not a micropower circuit.

Here is my receiver so far. With a 7.160MHz ceramic resonator it tunes to 7.000-7.075MHz that's 75KHz!
Without the varicap circuit connected the max frequency is about 7.105, but no tuning of course. I am testing it on breadboard, so I expect some capacitance from it as well.
The circuit operates on other HAM bands without any changes apart from the resonator.
If I manage to minimize the varicap capacitance then I can get higher range, so I see from the previous posts that the only solution is to use higher voltage.

I would really like a single (but low noise) transistor preamplifier before the audio amplifier to increase the audio volume a bit. Maybe an active LPF (http://www.sentex.ca/~mec1995/tutori...or6/xtor6.html Fig. 5) would be better but this has no preamplification. I am in the process of looking such a simple LPF but with gain added from a single transistor (things need to be kept simple). Any ideas are appreciated

You may like to try some different varicaps, they vary quite a lot in maximum and minimum capacitance.
I had to try quite a few different types to get the required tuning range at each end with sensible voltages. It involved quite a bit of messing about, and some patience, but well worth it.

I take it that your interest lies in the CW part of the band ?

I passed my morse code test about fifty years ago to get a full callsign. Never had a morse code QSO in my entire life. Once I passed the morse test I could not forget it fast enough.

There is a list here, though not quite accurate, so I checked the datasheets (the ones I could find).
Ceramic resonators in various ham bands, are more difficult to find than crystals, but on the other hand they combine a much wider tuning range (so you do not need many for each band) and adequate stability. So I really depend on the resonator pulling for covering different band parts, which hapened to be more on the CW range of the 40m.
The mv209 has quite low capacitance at 12v and it is cheap and readily available on ebay. But to fully extend the upper range you need higher voltage or some means of switched capacitors in series, which will of course decrease the whole tuning range a lot.

I regret I do not know morse code, because this would be great for these simple transceivers and QSOs would be computer-free. However I am too bored in learning it. Hopefully, when they changed the regulations here and discarded morse from advanced licencing, my novice licence was automatically converted to advanced, like I had passes the morse code exams.
So I was lucky in that sense, but unlucky I did not learn morse :)