Minimum Oscilloscope Bandwidth?

[mc1822]

I'm a new guy here but I do have electrical and some electronic experience. (Worked as an electrician & built numerous electronic kits) This is my first post on this forum and I hope to learn from posters on this here!                                                                                                                                                                                                                                                                                                                                                                           
I want to start repairing shortwave receiver radios that I might pick up at garage sales, etc. What is the minimum bandwidth on an oscilloscope that I would need for working on receivers  up to 30Mhz? My budget only allows me $275-$300 to spend. Would I be OK with a 100Mhz scope? 

[Grandchuck]

A 100 MHz scope should be good.

[DaJMasta]

Theoretically, you'd need a minimum of the top 30MHz channel plus the width of your upper sideband - but it's a relatively low requirement that should be covered by most modern scopes.  A 50MHz or 70MHz base level scope would be sufficient bandwidth for that application, but it's generally not much more for 100MHz inputs (and is often software upgradable or hackable on digital scopes), and that will cover you up into reasonable speed digital buses and keep square wave signals looking like square waves - since you really want little attenuation at the 3rd and 5th harmonics for a square wave to look basically square, that would let you see 20MHz square/pwm signals without the rise/fall time of the edge being too much of an issue.


There is a lot of discussion of budget scopes on this forum, so make sure to use that search button (the icon on the top right, not the box top left, it doesn't work as well) when you start thinking about a specific model.  Your budget is just at or under the low entry level price from many manufacturers, but there are some deals to be had under that or used scopes in that pricepoint that would probably be great choices - just make sure to look at reviews or people discussing the unit... on the budget end there can definitely still be stinkers.

[tggzzz]

Without knowing exactly what measurements you intend to make, I strongly suspect a spectrum analyser would be a more useful tool.

Make sure you understand the consequences of a scope's vertical axis specifications.

[bd139]

To fix short wave radios you don't really need a scope. You need an RF signal source, frequency counter and AC voltmeter. None of them have to be amazing specification so old second hand stuff is fine.

As tggzzz points out, knowing the measurements is important. Typically in a simple SW receiver you go through several stages on alignment:

1. stuff a low level RF signal into it (signal generator)
2. Tune it in and check the LO frequency with a counter and adjust.
3. Connect an AC voltmeter across the speaker.
4. Peak all the IF filters.
5. Adjust dial stringing if need be.

To fix them the process is similar but when something doesn't work, a signal injector/tracer is a lot more useful!

More complex radios, the radio test set comes in. Marconi 2955 does it all.

Not a scope in sight.

Not to say you can't use a scope but it's a jack of all trades instrument. If i had that use case in mind and budget I'd buy a junker 20MHz unit and the above tools as well.

[vk6zgo]

To fix short wave radios you don't really need a scope. You need an RF signal source, frequency counter and AC voltmeter. None of them have to be amazing specification so old second hand stuff is fine.

As tggzzz points out, knowing the measurements is important. Typically in a simple SW receiver you go through several stages on alignment:

1. stuff a low level RF signal into it (signal generator)
2. Tune it in and check the LO frequency with a counter and adjust.
3. Connect an AC voltmeter across the speaker.
4. Peak all the IF filters.
5. Adjust dial stringing if need be.

To fix them the process is similar but when something doesn't work, a signal injector/tracer is a lot more useful!

More complex radios, the radio test set comes in. Marconi 2955 does it all.

Not a scope in sight.

Not to say you can't use a scope but it's a jack of all trades instrument. If i had that use case in mind and budget I'd buy a junker 20MHz unit and the above tools as well.

It is often better (if less simple) to place a DC voltmeter across the receiver AGC line instead.
If you do use an AC voltmeter, keep the RF input as low level as you can, so the AGC doesn't mask the peak.

[David Hess]

An oscilloscope is not generally used to directly view RF signals but it might be used to view an RF envelope.  This requires either an oscilloscope of sufficient RF bandwidth or an RF demodulator probe which is simply an AM detector with a couple of diodes and capacitors.

So with a demodulator probe, you could get by with a 20 MHz bandwidth which will also be sufficient for audio and power supply troubleshooting.  In practice, you probably cannot find a good oscilloscope with less than 50 MHz of bandwidth.

My suggestion is to get a less expensive instrument so that you can gain experience with it to know what you really need.

[nigelwright7557]

100MHZ covers most generic projects.

You can get cheap USB scopes but beware of sampling frequency vs bandwidth.
Some have very unrealistic claims.
A 100 million sample/second usb scope is only good for about 2-3MHZ bandwidth.
I have seen some claim 30MHZ bandwidth with 100MHZ sampling !
This means you get 3 dots/points on screen  per sine wave cycle which is crap to say the least.

[vinlove]

Seen quite many handheld mini usb scopes with 5 Mhz bandwidths on ebay and Amazon recently for sale. And sime 2 Mhz ones with self assembly scopes for abot 20 usd. Do they have any use?

[EEEnthusiast]

you could also start with a RF power meter, RF signal source and frequency counter. A scope is useful for time domain debug, but not really for RF circuits. But it could come handy, if an oscillator on the board needs to be checked.

[nctnico]

This means you get 3 dots/points on screen  per sine wave cycle which is crap to say the least.

Nope. Look up sin x/x reconstruction and sampling theory in general. Usually digital scopes work well up to fs=2.5 * bandwidth. So 100Ms/s is good for 100/2.5=40Mhz. The theoretical limit is Nyquist but for practical purposes you'll need some headroom for the anti-aliasing filter.

Anyway, for repairing receiver I'd also look for a signal source (RF generator) and a DMM.

[RoGeorge]

Welcome to the forum.

A 100MHz oscilloscope will be fine.  This is a basic measurement tool for electronic work anyway, after the DMM (Digital Multi Meter).

However, for RF work you might also need an SA (Spectrum Analyzer) for example to look for spurious RF signals or other similar measurements done in the frequency domain, and/or a VNA (Vector Network Analyzer) for example to tune antennas, check filters and so on.

Expect either the SA or the VNA to be more expensive than an oscilloscope, in the range of $500 for used ones, unless you go for a poor's man SA or VNA (small USB only instruments).

[RoGeorge]

Was not meant to offend, sorry.   
Small like in handheld small, tools like NanoVNA which is about $50 (no idea if it worths the money or not).

[nctnico]

Was not meant to offend, sorry.   
Small like in handheld small, tools like NanoVNA which is about $50 (no idea if it worths the money or not).

I'm not sure whether a spectrum analyser or even a VNA is useful for fixing a receiver. These tools are more suitable for fixing / designing transmitters.

[Fungus]

Seen quite many handheld mini usb scopes with 5 Mhz bandwidths on ebay and Amazon recently for sale. And sime 2 Mhz ones with self assembly scopes for abot 20 usd. Do they have any use?

You mean these?



( starts at about 49:00 )

They're not useless, but $20 is about right.

[RoGeorge]

I'm not sure whether a spectrum analyser or even a VNA is useful for fixing a receiver. These tools are more suitable for fixing / designing transmitters.

That's true, for some reason I wrongly thought it was about repairing Rx/Tx radios.   

For vintage radios (receivers only), I don't even power the oscilloscope.  A schematic and a DMM is usually more than enough to repair those.  The most common faults are mechanical contacts in switches, dried out capacitors or audio power stages.

[thinkfat]

This means you get 3 dots/points on screen  per sine wave cycle which is crap to say the least.

Nope. Look up sin x/x reconstruction and sampling theory in general. Usually digital scopes work well up to fs=2.5 * bandwidth. So 100Ms/s is good for 100/2.5=40Mhz. The theoretical limit is Nyquist but for practical purposes you'll need some headroom for the anti-aliasing filter.

If you know your input is a sine wave (or generally, within the bandwidth limit), it's OK, yes. I think Dave published a video about DSO basics where he shows 4-times oversampling is the minimum for sin x/x reconstruction to work sensibly:

[Fungus]

Many people here owning fancy scopes use passive probes for everyday tasks, and those probes are practically up to 200MHz, despite higher rated BW (BW is good, but the input impedance will fall to below 50R, making them more or less useless).

Given the budget, just get a 1054, hack it to 100MHz (actually good up to ~150MHz) and enjoy it.

ie. Rigol DS1054Z.

As noted above though: Most of the failures will be bad switches, dried up capacitors, crusty power supplies, etc.

You may not even need an oscilloscope for most of the diagnosis, a DMM will do.

[vinlove]

Seen quite many handheld mini usb scopes with 5 Mhz bandwidths on ebay and Amazon recently for sale. And sime 2 Mhz ones with self assembly scopes for abot 20 usd. Do they have any use?

You mean these?



( starts at about 49:00 )





They're not useless, but $20 is about right.

yeah, mine came ready made in nice black plastic case.
I found it working well.

Scopes are for reading voltages in time duration, so that is what I am using it for.
I have one for 5 Mhz one too. It works even better. it was about 50 euros delivered.

[james_s]

More bandwidth is always nice, but for most hobby stuff I would agree that 100MHz is more than enough. Until the last decade or so a hobbyist was fortunate to have even a 20MHz scope, we are really spoiled in recent years. Many of the engineers designing the classic shortwave receivers in the first place would have had access to maybe a 15-30MHz scope at work. 100MHz scopes like the Tek 465 didn't appear until the mid 70s and they cost as much as a nice car at the time.

Regarding fixing radios, I don't think I've ever used my oscilloscope for that. Most radio faults are things like bad capacitors and cracked solder joints, dirty pots and drifted resistors. You should be able to accomplish most repairs with a multimeter and your eyeballs. Audio and RF test oscillators may occasionally be useful.

[nctnico]

This means you get 3 dots/points on screen  per sine wave cycle which is crap to say the least.

Nope. Look up sin x/x reconstruction and sampling theory in general. Usually digital scopes work well up to fs=2.5 * bandwidth. So 100Ms/s is good for 100/2.5=40Mhz. The theoretical limit is Nyquist but for practical purposes you'll need some headroom for the anti-aliasing filter.

If you know your input is a sine wave (or generally, within the bandwidth limit), it's OK, yes. I think Dave published a video about DSO basics where he shows 4-times oversampling is the minimum for sin x/x reconstruction to work sensibly:

Most probably sin x/x is broken on that Siglent scope from Dave's video (which is not an uncommon error on newly introduced oscilloscopes). On a decent DSO 2.5 samples per period is more than enough. DSOs have much steeper filters (far from a Gaussian roll-off) and most signals have harmonics which are multiples of the fundamental frequency so aliasing isn't a problem.

I've watched Dave's video again(number 1213) and the sin x/x mode on the Siglent scope (SDS1202X-E) he uses is definitely broken!

[mc1822]

Thanks to everyone that replied! I already have a digital multimeter, function generator, frequency counter and variable dc power supplies. I am curious if a audio generator or signal tracer would be of use?

[james_s]

It can be. For signal tracing something as simple as a small audio amplifier can be sufficient for radio work. It sounds more like you are accumulating toys rather than picking up tools with a specific task in mind. All the tools in the world are not a replacement for knowledge and experience. Radios are relatively simple and should not generally require anything more than a multimeter and soldering iron to repair. More specialized stuff like transceivers can require more specialized tools but most repairs are going to be simple.

The last radio I repaired had a bad speaker. The one before that had a cracked track in the volume pot. These were trivial to diagnose with a multimeter and a little intuition.

[tggzzz]

This means you get 3 dots/points on screen  per sine wave cycle which is crap to say the least.

Nope. Look up sin x/x reconstruction and sampling theory in general. Usually digital scopes work well up to fs=2.5 * bandwidth. So 100Ms/s is good for 100/2.5=40Mhz. The theoretical limit is Nyquist but for practical purposes you'll need some headroom for the anti-aliasing filter.

If you know your input is a sine wave (or generally, within the bandwidth limit), it's OK, yes.

Er, no. Arbitrary waveforms are fine, provided that the signal's highest component frequency is within the sampling limit.

It is, of course, necessary to be clear about the definition of "signal". For example a Tek 1502 has ~25kS/s and a 4GHz front end; the hp54100a has 40MS/s and a 1GHz front end.

[Fungus]

Thanks to everyone that replied! I already have a digital multimeter, function generator, frequency counter and variable dc power supplies. I am curious if a audio generator or signal tracer would be of use?

Sounds like you need to look at the Analog Discovery:

https://store.digilentinc.com/analog-discovery-2-pro-bundle/