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I think I have one of those....

you need to have a low distortion sine wave generator to go along with it.

As I recall (I'm at work so I don't have it in front of me), there are five knobs. One knob sets the amount of input to the meter, one knob sets the coarse frequency range and also has intermediate steps between the frequency ranges to set the 100% point, and one dial has the fine adjustment for the frequency range. I think there is also some kind of balance dial. Finally, there is a distortion range knob which has settings, in part, for 100%, 30%, 10%, etc., the other settings are something like 30V, 10V, etc. - this is because the HD-1 can also be used as a VTVM. In order to read THD, here is the basic procedure:

1) Connect the sine wave generator to what you want to test (device under test = DUT), connect the output of the DUT to the input of the HD-1. Before turning the HD-1 on, set the freq range dial to one of the in-between positions, turn the input dial all the way down, set the balance knob and fine frequency knob to a middle position, and set the distortion range knob to 100%. Then turn everything on, wait for everything to warm up (the HD-1 is a tube unit) and gradually turn the input dial up until the meter reads full scale. This sets the sensitivity of the unit. The next step will put in an adjustable notch filter, which you then adjust the remove the base frequency, and the meter then reads the residual harmonics, noise, etc.

2) Next, turn the freq range dial to the same frequency range as the sine wave generator, e.g. if the generator is putting out 1 kHz, turn the range dial to the input that includes 1 kHz in that range, say, 200 Hz to 2000 Hz (I'm making this up but you get the idea). When you do this, you should notice that the meter reads less than full scale.

3) Adjust the fine frequency knob to make the meter read as close to zero as possible. As the meter pointer swings down towards zero, you can switch the distortion range knob to lower scales to get a reading. Also, when you have the fine frequency knob adjusted for the lowest reading, adjust the balance knob and see if you can get it even lower. The two interact, so you may have to re-adjust a couple of times. As you get to lower scales, a small movement in the adjustment may result in a fairly large swing in the meter pointer. When you've got your adjustment optimized, you want the distortion range to be set so it gives the largest reading without pinning the meter needle. As I recall, the lowest distortion range is either 1% or 0.3%. The distortion is then read off the meter, using the scale that corresponds to the distortion range knob.

Using a Heath vacuum tube sine wave generator and an HD-1, the residual distortion (i.e. the lowest reading if you hook up the sine wave generator directly into the distortion meter) is about 0.05% IIRC. Hope this helps.


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  • I think I have one of those.... - JimL 12:13:16 06/23/05 (0)

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