[Elecraft] Tool/Equipment ?s...

[Tom Hammond NØSS]

Good morning John:

>I see Don's noise bridge kits mentioned often.  Why would I want one?  What
>would it do for me?

What John offers is not a noise 'bridge', but a noise 'source' (noise 
generator, if you will).

In all the books we've read from for decades, when we read discussions of 
adjusting filters, or setting BFOs to place the received signal properly 
within the filter, we are told to either use a sweep generator and an 
o'scope, to 'sweep' the signal thru the passband of the filter, or to use 
an accurately calibrated signal generator (often coupled to a freq. 
counter) to select precise frequencies on the edges of the filter, so that 
the filter can be adjusted to produce peaks at these frequencies.

All of these methods work well, but each required access to instruments 
many of us no longer have, of can readily find or borrow.

Since the xtal filters in the K2 (and K1, for that matter) are fixed-tuned, 
they themselves cannot be readily 'tweaked' for frequency response. 
However, we are provided with a means (CAL FIL) of easily shifting the BFO 
frequency to slide the received signal thru the filter passband. The trick 
it to know when and where to stop shifting.

A noise source generated a signal (well, noise) which a very broadband 
(from below 100kHz to well into VHF). Connecting this signal to the ANT 
input of the radio and then looking at output of the xtal filters (in the 
case of the K2, looking at the audio output with the sound card of your PC, 
using Spectrogram) will allow you to readily 'see' what the signal looks 
like as it passes thru the xtal filter and to adjust the BFO frequency in 
order to place it properly within the filter passband. It's a cheap and 
dirty (but very effective) method of setting up your radio's filters.

You CAN do the same thing using nothing more than the 'white' noise you 
hear on an empty (but not necessarily 'dead') band. The problem is that 
it's sometimes difficult to obtain an adequate level of noise to properly 
drive the sound card. Additionally, using band noise, you're always 
susceptible to variations in the band noise level and to someone else 
deciding to begin operating on the frequency you've chosen. The noise 
source, by the same token, will produce S9+ signal levels all the time, and 
since you're not connected to  an external ANTENNA, you'll only hear the 
noise, not other operators.

>Also, I see the occasional grid dip meter offered on eBay.  Why would I want
>one, and what would it do for me?

A grid dip meter/oscillator (GDO) is a variable oscillator which is 
connected to a tuned circuit and a meter to measure the level of the 
oscillation within the GDO. It's generally used to detect when an EXTERNAL 
tuned circuit is adjusted to the SAME frequency as that to which the GDO 
has been set. When the freq. of the external tuned circuit matched that of 
the GDO, some of the GDO's signal is absorbed by the ext. circuit thus 
reducing the level of oscillation on the GDO and causing its meter to 
reduce in reading (the 'dip').

GDOs can be used for tuned circuits, antennas (to see when they're in 
'resonance'), and similar uses. While it's a handy device, there are now 
other  devices available which seem to be better suited to the task. If you 
are a builder of other than kits (esp. where YOU design and build 
home-brewed devices), then a GDO may be of some assistance to you for 
getting tuned circuits 'in the ballpark'. If you want a GDO for tuning 
antennas, etc. then I'd suggest one of the much more accurate, and 
generally easier to use, antenna analyzers which are on the market today.

The GDO can also be used for getting a decent idea of the value of an 
unknown capacitor (if you have an inductor of a KNOWN value), or vice-versa 
(if you have a KNOWN cap). Put the two in parallel, measure their resonant 
frequency and the do the math to calculate the known device.

>Finally, there are always some monitor scopes offered on eBay.  These
>evidently let an operator look at his transmitted signal, and also at the
>received signal.  Why would I want one, what would it do for me, and can I
>hook one of these from Heathkit, Kenwood, or Yaesu up to my K2?

Yes! Though I'd warn against using a monitor scope to do much 'monitoring' 
of RECEIVED signals, it can be a wonderful(!) adjunct to your station, for 
monitoring your TRANSMITTED signal...! I've had a monitor scope for well 
over 30 years and would never be without one... period.

With a monitor scope watching your transmitted RF envelope, you can see the 
rise and fall times of the make and break, to enable you to tell whether 
you have a potential for transmitting key clicks. With a bit of experience, 
you can often 'see' the result of a transmitted parasitic oscillation (a 
'clean' RF envelope has a nice, sharp (green) top and bottom edge, but if 
there's a parasitic, or harmonic radiation, you'll generally see a fuzzy 
top/bottom edge and a secondary, dimmer, green line further down in the 
trace of the envelope). With a monitor scope, you can readily see 
fluctuations in the level of RF you're transmitting... something you might 
not catch on a meter, due to the ballistics of the meter needle, it will 
tend to average any tiny level  fluctuations which would be readily 
apparent from a scope trace... easy to see the effects of a poor connection 
somewhere along the RF path. You can even use a monitor scope to watch, and 
sometimes adjust, the weighting of your CW keying by first 'stopping' a 
single DIT on the display and then turning the sweep speed up to twice the 
rate. This will cause the image of the single DIT to 'wrap' around the 
display, so the make and break are superimposed one on top of the other. If 
you have 'perfect' 50% weighting, the make and break will just touch. If 
your weighting is a bit 'light', they'll not quite touch, and if the 
weighting is a bit too 'heavy' they'll overlap.

I warn against using a monitor scope for monitoring received signals 
because what you'll see is not only a representation of the actual received 
signal BUT ALSO a reflection of what your receiver might have done to the 
received signal as well. So if your receiver is not functioning perfectly 
(and giving you a true reproduction of the signal itself) it (the RX) can 
cause the monitor scope to see artifacts which are no really present on the 
received signal, but which are being generated INSIDE the receiver. This 
often leads to erroneous reports of 'bad signal' to the sending station, 
when it's the receiving station who is really at fault. We often place too 
much faith in our equipment being perfect, when it's really not.

You can place a well-designed monitor scope in the transmit RF path of just 
about ANY transmitter and benefit from what you see. Though it does take a 
bit of time and experience to be able to correlate what you see with what's 
actually happening... e.g. it's not always an immediate revelation.

Hope this helps.

73,

Tom Hammond   N0SS