By Bruce Stock – AB7YD
A sweep generator is a great tool for alignment of all sorts of things, including of course alignment of the IF of our Boatanchors. It is not a critical “must have” item like a signal generator, VTVM/VOM or scope, but it is a very handy addition that can tell you things that would be otherwise very difficult to see.
The hardest part of using a sweep generator is getting it properly hooked up to your scope, but I’ll defer that discussion for a bit, and start with a description of how a sweep generator works (and how a scope works too).
A sweep generator is basically just a voltage-controlled oscillator. A voltage is applied to an oscillator, causing it to sweep back and forth across a frequency range of interest, such as the IF passband of a receiver. See the diagram below.
The better sweepers use a voltage ramp to drive the Voltage Controlled Oscillator (VCO), and this results in a very linear sweep across the passband, followed by a rapid return to the starting frequency. Cheaper sweepers may use a sine-wave to drive the VCO resulting in a very non-linear sweep.
Sweep generators are easy to find, but good ones aren’t. Most of the sweepers you will likely encounter are the type made for TV IF alignment. These sweepers have a very wide sweep band (since a TV IF is 4.5 to 6 Mhz wide), and are usually swept at a 60 hz rate. For typical receiver IF alignment, we’d like a sweep bandwidth of around 5 to 10 Khz, and a slower rate.
The sweep rate is important because receivers with crystal or mechanical filters tend to take a short while to “ring up” to full output when set to narrow bandwidths. At a 60 hz sweep rate, some filters will never fully ring up, resulting in a much diminished peak on the scope. I have found that a sweep rate of 10-25 Hz is usually just right for most IF and crystal filter passbands.
The oscilloscope that you use with your sweeper is the other important part of the test setup. To see how the scope and the sweeper work together, first let’s look at how an oscilloscope works.
The CRT is driven by an X-Amplifier, which moves the spot horizontally, and a Y-Amplifier, which moves the spot vertically. The X-Amplifier is normally driven by the timebase and its associated triggering circuits. The timebase is basically just another ramp generator, the output of which is fed to the X-Amplifier. See the diagram below.
Some scopes do not have a way of getting directly to the input of the X-amplifier. You must have one that does if you are going to use it with a sweep generator. Those that have it usually have a position on the timebase sweep knob marked “X-Y”. When this position is selected, the timebase is disconnected from the X-Amplifier, and the input of that amplifier is made available …….somewhere. On some scopes it is accessible by a connector on the back panel; on others it is a connector on the front panel, usually in the area of the trigger inputs. On the more handy scopes it actually is made accessible via one of the Channel inputs. On my 2-channel Hitachi scope, for example, the Channel 1 input becomes the input to the X-Amplifier, while the Channel 2 input remains connected to the Y-Amplifier.
Now that you know how the sweeper and the oscilloscope work, it should be easy to connect them to sweep the IF of your treasured BA receiver. See the diagram below.
The Ramp Output of the sweep generator drives the horizontal, or X-Input, of the scope. The swept Signal Output of the generator is sent directly to the IF chain that you want to align. You usually pick up the output of the IF stages somewhere in the detector circuit, such as the secondary of the last IF transformer or at the detector diode itself. You should always set the receiver to Manual Gain rather than AVC, and adjust the Sweep Signal Output to the minimum amplitude that will give you a good trace on the scope.
The Sweep Generator will have some sort of “sweep width” control. This control adjusts the start and stop points of the sweep. Adjusting it will cause the passband pattern on the scope to expand or contract horizontally. Adjust the sweep width as required to get a usable pattern.
Well there you have it. The sweep generator is a wonderful way to get to see exactly what is happening when you peak up your next IF strip.
And finally, what if your scope does not have an X-input mode? You may still be able to use a sweeper if your scope has a “Ramp Output” from its timebase generator. Instead of using the ramp generator in the sweeper, you use the one in your scope, and route it into the sweeper to drive the VCO section. Or you build your own VCO, using a varicap, and drive it with the scope’s ramp output.