DMTD board

[tkamiya]

If I had a schemetic, I'd try mixing those frequencies in square wave and sine myself in my lab....  Maybe I'll just take a wild guess and try it.

I think, he's mixing 10MHz and 9.999999MHz, and get 1Hz, then feeding two of these to start and stop of interval counter each.  So RF and Local to each side of DBM via transformer, and center-tap goes to the counter?  I'll have to lower the frequency so I can use function generators rather than RF synthesizers.  I don't think that matters as long as difference is 1Hz.

That would be fun while I impatiently wait Coby's results....

I have few each of popular GPSDO, Rb, OCXO, and an old Cs in my lab.  What I lack is a good reliable reference and measurement method with fine enough resolutions.  I've tried comparing every permutations until I got to a point I started questioning my results.  Sure enough, results don't make much sense.  I'm trying to regroup myself (and lab) and hoping a new DMTD will fix some of the issues I had.

I looked up MC1450L and 1451L.  I really don't want to go eBay route as performance of these chips will be the key to success.  I found DigiKey had them via third party seller, but minimum quantity is like 30 pcs for close to $300. 

Anyone know what mixer Coby is using?  (Coby?  Hello!)

[thinkfat]

Not so easy to simulate at a 1Hz (or 10Hz) beat frequency. You'll want an FFT with sub-Hz bin width so that you can see all the mixing products, at the same time you have a 10MHz and a 10MHz+10Hz input, so you need to simulate at a quite high resolution. You'll need to generate a lot of samples to have enough waveforms at the beat frequency.

Anyway I don't think it matters. You get additional tones which "deform" the resulting waveform, but that's OK, since you'll be deforming it a lot more anyway with the adjacent limiter stages. What's important is that there the mixer will not introduce additional uncertainty (passive mixer has really good noise figure).

[tkamiya]

I have an HP3561A for FFT.  It can do sub milli-Hz resolution. 

I know it won't be an issue in Coby's implementation.  I'm just curious as questions were raised.

[5065AGuru]

Just a note,

The output of the mixer is filtered to remove any high frequencies and the BW of the op amp is around 35Hz So the 1Hz or 10Hz beat note should be suitably clean!

BOM finalized should post tomorrow.

Cheers,

Corby

[rubidium]

Regarding the discussion of square waves vs sine waves into a mixer, is it not true that we have become conditioned, i.e. by block diagrams of entities that have an "X" in them, to think that mixers act as multipliers of 2 waveforms in the strict mathematical sense when this is not the case with real circuits? To the contrary, I thought that most practical mixer circuits actually perform periodic inversion of the RF input based on zero crossings of the LO input, in which case the IF output is the same regardless of the LO being square (with 50% duty) or sine. Of course, having less jitter in the zero crossings of a square wave in noise over that of a sine in noise is the difference. I don't have any simple mixers lying around to play with, but I suspect that the IF output of one with a simple sine RF input and sine LO is more than just 2 lines, at (f_RF+f_LO) and (f_RF-f_LO), in the frequency domain due to this action. All of the odd harmonics of the LO frequency are mixed in there, regardless of LO waveform, so9 long as the zero crossings of the waveform are equally spaced. With suitable filtering, the fundamental (f_RF+f_LO) or (f_RF-f_LO) is what we care about.
Jim

[thinkfat]

I've attached a LTspice file that simulates a DBM with a beat frequency of 1000Hz. Maybe this will help for illustration. You can play with it, just set different amplitudes for V2. Start with 50mV, increase and observe the waveform between C1 and R2.

[notfaded1]

You're right Jim, zero crossing of square and sine wave at the same frequency is the same.

Bill

[5065AGuru]

Here is the BOM.

Schematic soon!

Cheers,

Corby

            Bill of materials for Simple Dual Mixer

All ICs DIP
2 MC1650L or 1651L
2 LT1008
2 LM311
2 74HCT14

Sockets
2 16 pin
2 14 pin
4 8 pin

2 SBL-1 mixers
1 circuit board
3 SMA to right angle PC connectors (long thread)

All capacitors radial .1" spacing
24 .1uf bypass
4 .1uf COG or film
2 .01uf COG
2 100pf

All resistors 1/4W physical sized 1% metal film
6 1K
12 51.1 Ohm
8 221 Ohm
2 100 Ohm
2 68.1 Ohm
3 16.7 Ohm


Chokes 1/4W physical size
4 3.9uh

All surface mount sized 1206 except noted
2 4700 pF COG
2 .0068 uF film
2 .1 uF film (1210) PPS dielectric (Digi-Key 338-3627-1-ND) for low TC
4 10uF Tantalum

2 1K
2 100K
2 47.5K

[tkamiya]

What is COG?

Is MC1651L necessary better for this application than MC1650L?

[rubidium]

What is COG?

Is MC1651L necessary better for this application than MC1650L?

C0G (AKA NP0) (note the zero, not an uppercase “O”) is an extremely stable dielectric, and is used to make capacitors that are not significantly affected by temperature, applied voltage, or aging. You will find that cheaper multilayer ceramic capacitors using barium titanate based dielectrics (like X7R or X5R) can turn out to have as low as 20% of their rated capacitance once sufficient DC biases are imposed over them.  When I learned that I got hooked on C0G's for critical applications. They cost more, generally require a larger volume for a given capacitance, and you're not going to find values greater than about 0.1uF. (If you do, be prepared to pay dearly). For something like 4700pF, the extra cost is not anything to be concerned about.
Jim

[tkamiya]

Oh....   I C....  C zero G....

I knew that...  cough, cough, cough....

[5065AGuru]

Both 1651 and 1650 tested the same so whatever is easier to source.

Cheers,

Corby

[Mrt12]

By the way, has anybody ever tried to use a Gilbert cell, like an NE612 or similar instead of a diode ring mixer?

[5065AGuru]

See change to BOM, corrected a capacitor value and added digi-key PN.

Cheers,

Corby

[tkamiya]

This is great.  I'm turning my eyes into sourcing them.

MC1650L and MC1651L, I found both at Digikey but they are from third party seller.  Nothing on Mouser.  Lots on eBay.  Any word on what to look out for?  I would imagine eBay stuff can be questionable.  I'd rather buy from someone reputable.

SBL-1, current models are SBL-1-1+ (0.1-400MHz, 7db), SBL-1+ (1-500MHz 7db), SBL-1X+ (10-1000MHz, 7db)
I would guess, first two are suitable?

Just requesting clarifications,
1)  last few items with K as units, they are registers, right?
2)  capacitors, 0.1 inch spacing:  what types are these?  ceramics?
3)  I'm guessing socketing ICs are not recommended, right?

[thinkfat]

MC1651L you'll have to buy from Ebay, Aliexpress or from Rochester Electronics, which will ask for your firstborn in exchange.

[thinkfat]

By the way, has anybody ever tried to use a Gilbert cell, like an NE612 or similar instead of a diode ring mixer?

I did some Googling for this, the only references that came up were time-nuts email threads (seems you were even involved in one ).

I guess they must be lacking some key qualities, noise and isolation come to mind.

[tkamiya]

I saw Rochester Electronics when I queried Digikey.  Do you know this company?  What is its reputation?

[thinkfat]

Oh the reputation is very good. They specialize in buying stock of end-of-life components and then selling them at a premium. You turn to Rochester Electronics when you're desperate to fulfill contractual obligations for military or aviation gear that cannot be upgraded or re-certified.

[tkamiya]

I've shot them an email asking if they would be willing to sell smaller quantities.  I don't need twenty of them.

[5065AGuru]

I bought 4 1650 and 4 1651 on eBay from US sellers and am satisfied with them.

Corby

[tkamiya]

Would you be comfortable telling me the seller's name?

[5065AGuru]

markltulsaok 1650L HAS MORE THAN 10 AVAILABLE FREE SHIPPING AND $6.75 EACH

The 1651L seller is out of stock.

There is another seller hodge16 that has 7 1651L available at $4.99 each and $3.99 shipping.

Cheers,

Corby

[5065AGuru]

Here is the schematic!

Also look back on page 1 for the "real" noise floor plot.

ExpressPCB file will post soon.

Once that last step is done I am going to start a new post detailing how to operate a DMTD system, showing how to get the best results and covering some of the little covered quirks and theory!

Cheers,

Corby

[tkamiya]

Corby,

What are you using for your offset oscillator?  It'd be difficult to shift OCXO for 5Hz, for example.  1Hz is possible with HP10811.  I was thinking of using my HP signal generator HP8644B, maybe.