Is a 7 digit frequency counter accurate enough for alignment?

[Ghetto]

Hey Guys,

I've seen a cheap 7 digit frequency counter on ebay. I've done a few searches on the net and see mixed arguments, some say its enough others say at least 8 digits to align HF radios. Whats your opinion?

This is what I'm interested in:

7 Digit counter that goes to 200mhz, no brand name for $25 + postage. Older looking unit or perhaps a project kit?:

FREQUENCY COUNTER - 200 MHZ - CB Radio / HAM Radio | eBay

<p dir="ltr" style="margin-top:0; margin-bottom:0;">FREQUENCY COUNTER - 200 MHZ</p> <br> <p dir="ltr" style="margin-top:0; margin-bottom:0;">Working condition!</p>

www.ebay.com.au

For example I have a HR2510 that I want to align.

 

[Pez]

If it works, and if it is sensitive enough to see the signals you are trying to measure, and if it is accurate enough then it would be "OK". I say OK because it will get you close enough to align a HR2510. (You could have 20 digits, but if the resolution and accuracy is still low, then the number of digits becomes irrelevant), - all unknowns unfortunately. It is probably not a good choice for all the reasons above, but for $25, its cheap!

 

[nomadradio]

Looks like a high-school science-fair project.

Might be solid and stable.

Maybe. I would place more trust in one of the chinesium counters on fleabay. Many of them have a temperature-compensated time-base oscillator. I'd be surprised to find that feature in the home-brew counter. Biggest hassle using a frequency counter for alignment is to find a point in the circuit where the crystal you're measuring is isolated from the crystal itself. A so-called "direct" probe will connect the probe tip to the full 100 to 200 pf of capacitance between the shielded wire's center conductor and the shield. Better policy is to use a probe meant for oscilloscopes either a times-ten (X10) that attenuates the input signal by a factor of ten-to-one or one that's switchable. Main benefit is that this reduces the circuit loading when you touch the probe to the circuit being measured. Doesn't eliminate it, just reduces it to a more-manageable level of circuit loading. Just one problem. The counter's input sensitivity must be pretty high, so it can display a steady reading when probing a low-voltage circuit.

Some 'scope probes are switchable, either times one or times ten. A tech is frustrated trying to get a stable reading with the probe in the times ten position. Just not enough signal level to stabilize the display. Switches the probe to times one and gets a steady reading. Just one problem. When the probe is unhooked from the circuit, the frequency will jump up from where it was adjusted with the probe connected to the radio. Removing the capacitor to ground inside the probe and cable made the adjustment irrelevant.

Not all counters are created equal. The sensitivity of the counter's input should be adjustable on the front panel, since driving the counter's input too hard can result in a bogus reading, too.

I'll skip the notion of a preamp on a counter's input for now. Getting a measurement you can trust takes more than just a frequency counter and probe. The setup has to be right, too.

73

 

[Ghetto]

Thanks guys as always. Thanks Nomad for the extra info as well. Was wondering about all that.

I think I'll pass then because that's my worry: how do I even know its accurate or would work for what I want to do.

I guess I'm always looking for a bargain!

 

[Captain Kilowatt]

Do not confuse accuracy with resolution. Accuracy is how close to reality the display indicates, while resolution is how fine the result can be determined. A 7 digit counter may be extremely accurate but if it has low resolution.....only to the nearest 100 hertz.......then it is NOT acceptable for proper alignments IMHO. I say this assuming the 7 digits are in the format of xxx.xxx.x since it covers above 100MHz.

 

[Dr_DX]

8 digit minimum with resolution down to at least 10Hz; preferably 1Hz.