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  #1  
Old 01-21-2017, 05:50 PM
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Zenith6S321 Zenith6S321 is offline
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Downconvert Sweep Gens for early TVs

We can use 45.75 MHz sweep alignment and marker generators to align the early low band IF (18MHz to 38MHz) TVs if we shift their output frequency down (downconvert) to what the TVs need. The parts to do this are easy to get. I made one using a Mini-Circuits TUF-1+ mixer, a cheap DDS (direct digital synthesis) sine wave generator with LCD display and keys, and an RF amplifier module. The parts cost me about $50.00. Attached are a couple of pictures of my B&K 415 low band setup. Here is a list of the components and links to a source (not mine).

Mini-Circuits TUF-1+
PDF: https://www.minicircuits.com/pdfs/TUF-1+.pdf
Source: https://www.minicircuits.com/WebStor...model=TUF-1%2B

Sine wave board source: http://www.ebay.com/itm/AD9850-DDS-F...sAAOSwVL1V-OSt
or
http://www.ebay.com/itm/AD9850-modul...sAAOSwwPhWkcvf

RF Amp board source: http://www.ebay.com/itm/1PC-broadban...sAAOSwo4pYG~A4
or
http://www.ebay.com/itm/Broadband-RF...gAAOSwEzxYQ7z3

The downconverter is connected between the sweep and marker generator RF output and the TV. The TUF-1+ mixer RF input is driven by the sweep and marker generator RF output. The TUF-1+ local oscillator (LO) input is driven by the sine wave generator. The TUF-1+ IF (IF) output drives the RF amp board input. The RF amp output drives the TV sweep gen input point. I may replace the RF amp output coupling capacitor with a higher voltage one, just in case.

I will try to post some more pictures and waveforms, but I have to delete some old VK posted pictures to make room for them.
Attached Images
File Type: jpg downconverter.jpg (72.5 KB, 37 views)
File Type: jpg B&K415-LowBandSetup.jpg (119.4 KB, 37 views)

Last edited by Zenith6S321; 06-03-2017 at 07:49 PM.
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  #2  
Old 01-21-2017, 06:16 PM
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Zenith6S321 Zenith6S321 is offline
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The first picture is the spectrum of the B&K 415 RF output from about 38 to 49 MHz. Its varies about 3 dbV.

The second picture shows the downconverter output with a 20MHz sine wave to shift the 45.75 MHz sweep down to 25.75 MHz (45.75 - 20 = 25.75). Other harmonics show up, but the TV IF only amplifies its IF range signals (19.75-27.25 for this case). This downconverted output also varies by about 3 dbV.

If your old TV has a different IF range you calculate the value to enter into the sine wave generator by taking the 45.75 - your_TV_video_carrier_frequency = sine_wave_frequency. In the case of this TS721 its 45.75 - 25.75 = 20MHz.

The third picture shows the display of the B&K 415 scope XY outputs with the downconverter RF connected to an RCA 721TS tuner converter sweep input test point and with a direct probe (a scope probe set to 1X) connected to the video detector sweep test point. Yes, the B&K 415 markers all work since they are added by the B&K after the signal comes from the TV and the B&K then drives the scope H&V inputs.

The scope I am using is my new Digilent Analog Discovery USB scope. It's limited to a 50 MHz bandwidth, but it includes a bunch of features like a spectrum analyzer (FFT), a logic analyzer, an I2C and SPI serial protocol analyzer and a lot more. Here is a link if you are in the market:
http://www.digikey.com/en/product-hi...FRGBswodfAoO-Q
Be sure to get the BNC adapter as it expands the input frequency range when using scope probes instead of the fly-leads:
http://www.digikey.com/product-detai...073-ND/4840860
I thinks it's a great value for $280.

The fourth picture is the IF spectrum input to the video detector. It makes it easy to get the traps tuned to exactly 21.25MHz.
Attached Images
File Type: png B&K415-Output-Spectrum.png (78.7 KB, 18 views)
File Type: png B&K415-20MHz-Mixer.png (71.2 KB, 13 views)
File Type: png B&K415-721TS-Sweep.png (86.7 KB, 15 views)
File Type: png B&K415-Output-Spectrum-721TS-DetectorIn-2.png (66.6 KB, 12 views)

Last edited by Zenith6S321; 01-21-2017 at 06:23 PM.
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Old 01-21-2017, 06:58 PM
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Interesting. If it holds up to practical alignment work I'd consider building it in and adding a switch.
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Old 01-21-2017, 08:33 PM
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We used a similar setup in the Navy to check center frequency on TACAN flightline test sets and 243MHz Emergency beacons. Both were swept frequency output, making a freq counter unusable, so we heterodyned/down converted the output with another frequency (offset by 10-60Mhz). Both went into a simple diode mixer, and the difference was either displayed on a scope, spectrum analyzer, or fed to a frequency selective voltmeter.

Very common usage back in the 50s-70s, before GHz-capable spectrum analyzers, scopes with 500MHz (+) bandwidth, and DSP filtering.

Cool to see the setup for TV IF Sweep!

Someday I'll get the gumption to do your VA62 mod, haven't the need for it yet.
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Last edited by Findm-Keepm; 01-21-2017 at 08:49 PM.
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Old 02-03-2017, 10:32 AM
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Comparing B&K to other sweep gens

I did an alignment using the B&K415 45.75 MHz output down converted by 20 MHz to generate the 25.75 MHz IF needed by the RCA 721TS. I then took pictures of the B&K415 scope sweep display, a Wavetek 1002 scope sweep display, a 25.75 MHz IF frequency VA62A 10 bar multiburst (0 MHz to 4.5 MHz detected video), and the CRT image of the VA62A multiburst. I used the same 75 ohm to 300 ohm matching pad (from the B&K415) on all of the equipment to connect their RF outputs to the 721TS tuner sweep alignment input test point. I also used the same 1X scope probe connected to the 721TS sweep alignment detected video test point.

The first picture is the down converted B&K415 scope sweep display. the markers are 21.25, 22.75, 23, 24, and 25.75 MHz (down converted by 20 MHz).

The second picture is the Wavetek 1002 set for sweeping the 18-28 MHz range with 1 MHz markers.

The third picture is the VA62A set to produce a 25.75 MHz IF video carrier frequency driving the same 721TS tuner IF sweep test point with the 10 bar multiburst pattern. The scope waveform of the detected video shows the sequence of bar frequencies 0, .5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, and 4.5 MHz. The frequencies are shown on the scope waveform as the low to higher frequency square waves. By looking at the amplitude of each you can see the frequency response of the corresponding IF frequencies. 0 Mhz = 25.75, .5 = 25.25, 1.0 = 24.75, down to 4.5 = 21.25 MHz.

The fourth picture is the CRT image of the VA62A multiburst. It shows the vertical lines that the multiburst pattern produces on the screen. The brightness of the vertical lines is the maximum voltage on the scope multiburst waveform. The solid wide bar at the left is 0 Mhz, next is .5 MHz, and so on across the screen. So you can that the amplitude is about the same from 0 to 2.5 MHz but drops off at 3.0 MHz since that bar of vertical lines is dimmer the those to the left. This image on the CRT directly corresponds to the amplitude of the scope multiburst bars as well as the Wavetek IF sweep amplitude and the down converted B&K415 sweep amplitudes.

I am convinced that the down converted B&K415 works as well as the Wavetek 1002 as an alignment sweep generator. The VA62A is also useful ONCE YOU GET A GOOD SWEEP ALIGNMENT. You can the use its CRT image of the multiburst to tweak the IF adjustments that correspond with the multiburst bar frequencies to touch up the total tuner input to CRT image frequency response to what you are after. I have adjusted this 721TS for the RCA alignment procedure. But in the past I have also used the VA62A CRT multiburst patter to adjust the response to show bars out to 4 MHz. That resulted in a sweep pattern that showed higher IF amplitudes at the 22 MHz end of the spectrum. It also resulted in an overall lower receiver sensitivity. I guess its the gain-bandwidth product problem. You can have higher gain with a lower bandwidth, or a higher bandwidth with a lower gain.
Attached Images
File Type: jpg BK415-DownConv.jpg (52.9 KB, 17 views)
File Type: jpg Wavetek1002.jpg (45.1 KB, 14 views)
File Type: jpg VA62-ScopeMultiburst.jpg (45.6 KB, 19 views)
File Type: jpg VA62-CRTMultiburst.jpg (63.8 KB, 19 views)

Last edited by Zenith6S321; 02-03-2017 at 10:36 AM.
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  #6  
Old 02-03-2017, 11:26 AM
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I may have to buy up the parts for this and get cranking on rebuilding my 415.
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