A Look at VFO Drift in the MFJ Cub^TM

Larry East, W1HUE
Tucson, Arizona
(contact author)

Some Cub owners have noted significant warm-up frequency drift, whereas others see very little drift. Such variations are to be expected due the normal production tolerances of the parts used in the Cub VFO.

The major sources of possible drift in the Cub VFO besides the inductor and capacitors used in the circuit are the "tuning diode" (D2, an MV2104) and the SA602 mixer (U2) which is also used as the oscillator. The thermal drift properties of tuning diodes vary as a function of applied voltage and hence capacity. A tuning diode can be compensated by using an appropriate series diode and shunt resistor, but the usual approach is to reduce its influence on frequency drift by making it a small part of the total circuit capacitance. (This is the case for the Cub, so don't worry about it.) Slug tuned inductors (and inductors on wound powdered iron toroids) usually have a positive temperature coefficient, causing the oscillator frequency to decrease with temperature. Most capacitors have a negative temperature coefficient, so the proper choice of capacitor size(s) and type(s) will almost exactly compensate the temperature drift of the inductor.

Unfortunately, the thermal properties of '602/612 mixers vary from device to device making it close to impossible to come up with a drift cure that will work for everyone. For this and other reasons, it is preferable to use a separate FET or bipolar oscillator for VFO's. This was not done in the Cub, probably to keep the parts count down. Therefore what works to stabilize my Cub may actually increase the drift in yours!

In its "stock" form, my 15M Cub would drift down about 2 kHz during the first 10 minutes after a "cold start" and another 300-500 Hz over the next 5 minutes. It then settled down and would maintain a set frequency within a couple of hundred Hz or so over the next hour or two – provided the ambient temperature remained relatively constant.

Since temperature compensating a VFO can be a long and frustrating exercise, I tried to convince myself that this was "good enough". However, after several QSOs during which I was continually "tweaking" the dial, I decided to bite the bullet and see if I could improve the situation. Well, I was very lucky; after a few hours (rather than a few days) of diddling with different capacitor combinations, I was able to reduce the drift to almost zero! The VFO now wanders up and down about ± 50 Hz during the first 10-15 minutes of warm-up, and then changes no more than ± 25 Hz during any 10-minute period. In one test, I left it running on the bench for four hours and the total drift was only 90 Hz. It did drift up and down some during that time, but never the less, this is a BIG improvement over the original drift!

The improvement was achieved by replacing C6 (originally a 150pF "mono cap") with three caps in parallel: A 100pF NP0 mono ceramic, a 10pF N900 disc ceramic and a 39pF NP0 disk ceramic (the two disk caps are soldered on the bottom of the PC board). For those not familiar with capacitor designations, "NP0" caps have very low temperature drift; typically less than ± 100 PPM per degree C. (The "mono" caps used in the Cub are supposed to be NP0.) "N900" means that the drift is "nominally" a negative 900 PPM/°C.

As explained above, this combination of capacitors may not work for your 15M Cub, and certainly not for other band models.

If you want to try to reduce the drift of your Cub (or any other rig), here are a few things to keep in mind:

You won't be able to make any meaningful drift measurements immediately after touching a soldering iron to the rig. Let it sit with the power OFF for at least 30 minutes (preferably an hour) before making drift measurements.
The 80M and 40M Cubs use "down conversion", meaning that the output frequency drift is in the opposite direction to the VFO drift.
Some capacitor types – especially ceramic caps that have sat in a parts bin for a few years – need to be "conditioned" for several minutes to an hour or more in an operating circuit before they completely stabilize.
The best way to check for drift is with a frequency counter. You can also monitor the VFO on another receiver, but this is less precise. In any event, make sure that your measuring device is relatively stable; trying to check for VFO drift using a receiver that also drifts is a rather hopeless endeavor!

Here's some information on the drift characteristics and availability of some common types of capacitors:

NP0 caps have a temperature coefficient that is ideally zero, but in practice they may exhibit small positive or negative coefficients (more likely negative in my experience). C0G caps are the same as NP0; C0G is an "EIA" designation for NP0 type. Most parts suppliers (Mouser, Digi-Key, etc.) carry a large variety of NP0/C0G capacitors, both "monolithic" and disk.

"Nxxx" on a capacitor specifies the nominal change in capacitance with temperature; N750 means -750 PPM/°C, N1500 means -1500 PPM/°C, etc. The value is not exact and there will be some variation between capacitors carrying the same designation. These types may also carry an "EIA" designation; the more common ones are:

P2G → N150

T2H → N470

U2J → N750

P3K → N1500

Nxxx disk ceramics may also be designated by a colored dot on the top:

Black → NP0

Red → N075

Orange → N150

Blue → N470

Violet → N750

Digi-Key carries Philips N750 ceramic disk capacitors [they have a specified temperature coefficient of (-1000 ± 120) PPM/°C]. Other Nxxx caps can often be found in capacitor assortments from surplus dealers and occasionally in Radio Shack capacitor assortments. They are also sometimes available from Dan's Small Parts.

Polystyrene capacitors – used in the Cub VFO Cub in most models – have a negative temperature coefficient of 150 PPM/°C. These caps are physically larger than ceramic caps and are easily damaged by heat. Do not confuse these with polypropylene capacitors which, although designated as "stable", do not usually have a specified temperature coefficient. Mouser carries an assortment polystyrene caps.

Silver mica capacitors are often referred to as "high stability capacitors", but they do have a small positive temperature coefficient. In fact, silver micas are about the only commonly available capacitors that have a reproducible and fairly linear positive (but usually unspecified) temperature coefficient. They suffer very little from aging effects; hence they are "highly stable". Silver mica caps can be obtained from Mouser and Dan's Small Parts.

You may occasionally find disk ceramic caps marked "P100" indicating that they have a positive 100 PPM/°C. temperature coefficient (these may also be marked with a white dot on top).

X7R, X7U etc. capacitors should not be used in a VFO even though they are usually listed as "temperature stable". They have low temperature drift around room temperature, but their drift can be positive or negative at extended temperatures.

Z5U caps should never be used in frequency determining circuits; they are intended for coupling and bypass use only.

If your Cub VFO decreases in frequency during warm-up, replace a poly cap used for C6 or C7 with an equal value combination of NP0 and Nxxx (or poly) caps. If the direction of drift reverses, use a larger poly (or Nxxx) cap and smaller NP0 cap. If the drift is still downward in frequency, try changing both C6 and C7 to combinations of NP0 and poly or Nxxx caps. You can also replace mono caps used for C6 (or C7) with combinations of NP0 and Nxxx (or poly) caps to reduce a negative frequency drift (this is the approach that worked for me).

If your VFO increases in frequency during warm-up, replace a ploy cap used for C6 or C7 with an equal value NP0. If the drift direction reverses, try combinations of NP0 and poly (or Nxxx) caps to minimize the drift. If it is still positive, try silver mica caps in combination with NP0 (or poly) caps.

A Look at VFO Drift in the MFJ CubTM

A Look at VFO Drift in the MFJ Cub^TM