SDG1025 sweep and Rigol DS1054Z - Page 1
Forum section: electronics / beginners

author: smbaker
author_rank: Regular Contributor
timestamp: November 16, 2015, 01:05:23 am
content: I'm using an SDG1025 to align an FM radio. To do this, I use sweep centered on 10.7 Mhz with 450 Khz width. The SDG1025 is sweep is set to output trigger pulses, and I have thr Rigol configured to trigger on the downward edge of the SDG1025's sync output. This means the Rigol triggers in the middle of the sweep. This produces a signal on the radio's audio output that looks like figure #1 in the attached screenshot.For alignment, the manual wants me to display this waveform on top of itself, 180 degrees out of phase, so that the upslopes and the downslopes cross. This would roughly be #2 in my attached screenshot.People in the radio forums tell me that with older equipment, this would have all just worked, since the older equipment usually sweeps using a whole sine wave. Sweep at 60Hz, and set the scope to 120 Hz. I'm trying to figure out how to duplicate this using my SDG and DSO. I've tried outputting a 120Hz sawtooth on the second channel of the SDG, and using that to horizontally deflect the Rigol, and I was able to get something that approximately what I was looking for, but the resolution was just awful, and I couldn't see any detail.Any tips?

author: smbaker
author_rank: Regular Contributor
timestamp: November 16, 2015, 04:49:35 am
content: Playing more with the SDG1025, I did figure out that I can setup a sine wave on channel 2 and use it to FM modulate channel 1. This gives me more-or-less what an old-time tube era RF generator might have done. If I hook it up to a scope in XY mode, then I think it's going to give me the plot the radio repair manual was looking for.Fortunately, I have an old analog scope that I can try out. I just don't think the DS1054Z is going to cut it for XY plotting.

author: AF6LJ
author_rank: Unknown
timestamp: November 16, 2015, 05:06:58 am
content: Just a few thoughts.How hard you are driving your IF strip will effect what you see.How are you coupling the 10.7MHZ into the IF strip..Is the FM radio tube or transistor?

author: smbaker
author_rank: Regular Contributor
timestamp: November 16, 2015, 05:27:08 am
content: It's a tube radio. I'm following a Sams photofact that has me connecting the output directly between chassis and pins on the IF tubes. The photofact doesn't specify what voltage should be used. I started out low, at 100mVpp, and then went up to 1Vpp, where I seem to get a decent result.I was able to perform the first three alignment steps in the photofact without much issue. It's the 4th alignment step that's looking for this crossover waveform where I got stumped trying to figure out how to configure the signal generator and scope.

author: AF6LJ
author_rank: Unknown
timestamp: November 16, 2015, 02:00:07 pm
content: Quote from: smbaker on November 16, 2015, 05:27:08 amSweep generators used back in the day did use sinusoidal sweep, usually at the line frequency making it easy for the scope to synch to.You can use modern equipment, I am assuming that somewhere in the text they mention the use of a marker or marker generator which would be tuned to 10.7MHZ and maybe adjustable to plus and minus 75 to 100KHZ. in order to find the edges of the IF passband. Somewhere in the garage is a B&K sweep generator made for the purpose.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 17, 2015, 08:08:47 am
content: Modern sweepers use a sawtooth driven sweep,At the beginning of the ramp portion the swept frequency is lowest.The swept frequency increases with the ramp amplitude,until at its peak,the frequency is highest.During the fast retrace of the driving waveform,the sweep returns to its lowest frequency.Such a sweeper may produce its own sawtooth,& use that to deflect the host Oscilloscope directly in the XY mode,or it may trigger the 'scope's own timebase.Alternatively,it may steal a "sawtooth" signal from the Oscilloscope.Many "old school" sweepers used sinewave driven sweeps.Starting say,at the negative peak of the driving waveform,the swept frequency increases, reaching its highest figure at the positive peak,then sweeps down again,until at the next negative peak,it is at its lowest frequency.Each cycle of the driving waveform thus produces two full sweeps,low to high,followed by high to low.Generally,to prevent the swept frequencies being displayed unevenly,it is necessary to have the same sweep waveform for both the sweeper & the X sweep of the 'scope.This is not so important for the discriminator display,however,so a sinewave driven RF sweep,can be used,in conjunction with a sawtooth X sweep on the 'scope.With the X timebase at twice the frequency of the sweep drive,it produces the "crossover" display you are trying to obtain.The main reason for using this display is that it allows the discriminator response to be centred upon the IF frequency correctly.Modern generators are either accurate enough or have markers so that centreing is not a problem.This link gives a good description of the old time devices.http://www.radiomuseum.org/forumdata/users/4942/file/FM%20Alignment%20Ghirardi%201952%20OCR.pdf

author: smbaker
author_rank: Regular Contributor
timestamp: November 18, 2015, 01:38:51 am
content: Quote from: vk6zgo on November 17, 2015, 08:08:47 amThere's no way for me to get the crossover pattern without putting the scope in XY mode, is there? On a digital scope even if I double the X timebase, I'm still only going to get one trace of the waveform on screen at any given time, right?Fortunately I do have an old analog scope that I can try, but I'd have really liked to be able to do it all with the Rigol.I did order myself an old tube RF sweeper from ebay in 'tubes light up but I don't know how to test it' condition (i.e. almost certainly will need fixing). It has a built in marker feature. It'll be fun to use equipment closer to the era of the radios themselves.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 18, 2015, 03:38:49 am
content: You don't really need the crossover display--it was really just a workaround of the limitations of old time equipment.Non-triggered 'scopes could really only "sync" at multiples (& sub-multiples) of the signal input to the vertical amp,so 2X was a good choice.I don't think a triggered analog 'scope will display it properly,due to the retrace blanking which is normally used,nor will a DSO,for the reason you quoted.As Sue pointed out,if you can generate a 10.7MHz marker,you can do this adjustment with modern equipment.There are several Youtube videos showing this.You can check if the old sweeper is at least producing a signal,by looking at its RF output with the DS11054Z.If you make up a LC tuned circuit & hang it across the cable between the two,you should see its resonant peak.The Rigol 's FFT should also be able to see the thing sweeping.