[ARC5] BC-221 and LM on eBay

[Bob kb8tq]

Hi

The answer is very much a “that depends” sort of thing.

The internal crystal isn’t temperature compensated. If you zero it today indoors
and use it outdoors a month from now, there will be an error. If you let it warm
up / stabilize for a couple days, zero it, and then use it … maybe not much error 
from the crystal.

A lot of stuff with one of these depends on zero beats. Just how close you can 
get on a zero beat depends on multiple things. How good are your headphones?
Do they go deaf at 200 Hz? Can you still hear a rumble at 20 Hz? Are the levels
such that you can get a “swish swish swish” sort of beat note well below the
audio? How close can you guess between the two sides of a 20Hz beat note? 

First step is to sync the crystal to WWV. 20 Hz is a reasonable thing. Fractions of a 
Hz are not at all easy. Is you lab temperature stable enough / your crystal good enough 
for that to matter? Maybe it is … maybe ….(yes, this is *lots* easier with a BC-221 that 
lets you easily get to the crystal tune cap from the outside ….). 

What WWV-ish thing are you checking against? 20 Hz at 5 MHz is 4 ppm. At 20 MHz
it would be 1 ppm. All the transmissions are equally accurate so higher (if you can 
hear it) is going to be “better” in this case. 

If you are at a zero beat “cal point” you are right back to how close that zero beat is.
Since the cal points are harmonics of this vs harmonics of that, the math of how close
20 Hz is a bit harder to work out.  ( Yes there are “unlisted” cal points that can be 
used, that gets really crazy really fast).

With your 4 MHz signal, is that a transmitted signal or a received signal? You may
have yet another “how close can you zero beat?” tossed into the mix. 

Does this all add up to <5 ppm accuracy? Probably not, though it might. Something 
closer to 10 or 20 ppm is likely the practical limit once rest of the mechanism gets 
into the act. Again, this is in a bench environment with a lot of care being used. 

Bob

> On Mar 28, 2022, at 10:24 AM, MICHAEL ST ANGELO <mstangelo at comcast.net> wrote:
> 
> I picked up a BC-221 and a neighbor gave me a LM frequency meter. He was a high school physics teacher and one of his students gave it to him.
> 
> What is the published frequency accuracy of these meters? The only notes I could find online is this observation by VE3BDB. He read in the manual that that the maximum error will occur when measuring a 4 MHz signal at minus 30 degrees C where an error of 1.355 KHz (0.034%) could occur. His observations indicated that the BC-221 is easily capable of 250 Hz accuracy on 80 & 40 M.
> 
> <https://www.qsl.net/ve3bdb/bc221observations.htm>
> 
> Mike N2MS
> 
> 
>> On 03/28/2022 8:38 AM Bob kb8tq <kb8tq at n1k.org> wrote:
>> 
>> 
>> Hi
>> 
>> As an “all in one” gizmo for keeping a pile of HF radio gear tuned up,
>> I still would put the LM / BC-221 pretty close to the top of the list. Yes,
>> there would be other things on the list as well. With care, you can do a 
>> lot of things with one of them.
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