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August 2007 Archive

 

31 August 2007

I've added phase noise data for a Z90 operating in signal generator mode. Since the Z90's AD9851 DDS circuitry is quite similar to the DDS-60, the Z90 data can be taken as representative of a DDS-60. The data is at the Canned Osc Phase Noise page.

 

30 August 2007

I've been intrigued with the small custom one-time programmable oscillator modules, such as Cardinal Components' CPP series units. http://www.cardinalxtal.com/docs/series/CPP.pdf. The CPP series is a one-time programmable module, with 1 Hz steps and is modestly priced at $8.18 each from DigiKey, programmed to your frequency.

If, and it's a big if, these modules have acceptable phase noise they would be extremely handy for BFO or 2nd oscillators in one-off or low volume designs. A few months ago, I purchased several of these oscillators for 21.4 MHz and was disappointed at their broadband noise output. I've added a new page to this site comparing the CPP oscillator with a standard canned oscillator and a home brew crystal oscillator. You can view the new page by clicking here or via the links at the top left of this page.

 

27 August 2007

I've finished laying out an interim printed circuit board for the RF section of my next generation panadapter. It took a full week to enter the schematic, make patterns for parts not in the stock libraries and lay out the PCB. I plan to order prototype boards later today.

I've also laid out a test board for component measurement with a VNA. The board has provisions for through measurements as well as reflection measurements. I will have a few extra boards and can supply them to interested readers while the supply lasts for $15.00 each, including domestic US shipping. This price includes the PCB, and 1206 parts for the short and 50 ohm load. The board also requires 8 "V-Bite" Teflon BNC connectors available for purchase from Mouser at about $3.50 each.

I'll post more details, including photos and test results when the boards arrive and I verify the layout.

21 August 2007

I've looked a bit more at Vista support for the E-MU 0202 USB sound card. This is of academic interest to me at the moment because I don't have a Vista machine, but at some point I will almost certainly be forced into one as new XP installations will come to an end before long. The short answer is that there's an existing work-around to install E-MU 0202's XP drivers in Vista and the release data for real Vista drivers is 3Q 2007. I've added this data to my Softrock page and provided links.

 
20 August 2007

My wife uses a Taylor digital oven thermometer (model 1470) to measure the interior temperature of roasts and the like. The first one got a bit too close to a hot stove burner and the case melted, so I replaced it with a new one earlier this year.

Recently, she remarked that the new thermometer was reading differently than the old one, so today I compared the old and new oven thermometers along with two glass "laboratory" thermometers, two analog dial thermometers and my Fluke 189 Digital Voltmeter and 80PK-25 Type K thermocouple combination. I've added a new page with the results of my comparison, viewable by clicking here or via the links at the top left of this page.

If you don't have one, by the way, I recommend the Taylor 1470. As my measurement data shows, both samples I tested agree well with my Type K thermocouple.
 

19 August 2007

The last few days have been occupied by entering a schematic of the RF section of the "Z93" next generation panadapter into DIPtrace's schematic capture program. This is a fairly time consuming process, but not nearly as painful as doing the associated PCB layout. I hope to have a PCB layout done by the end of the week and enter an order for a couple prototype boards. This represents a modest step forward, and is mostly to verify the breadboard work I've done this month in the areas of sensitivity, IP3 and IP2 performance, gain distribution and the like. I expect to make quite a few changes in this design as the project goes forward. I can't start on the rest of the work�the DSP section and the display/controller�with the RF as a selection of Manhattan-style boards spread over the test bench, however. The next major element of the project I will work on is likely to be the display and main microcontroller, assuming the RF section is not in too bad a shape.

Larry, N8LP, purchased an E-MU 0202 USB sound card at my recommendation a few days ago and a comment he made alerted me to something I had overlooked. The E-MU 0202 ships with only Windows XP drivers, and has no support for Windows 2000. It also does not support Vista, but that's a short-term issue as far as I can tell.  I'll add a note to my Softrock page noting the E-MU 0202's compatibility limits.

If you are interested in WW II era aircraft radio communications gear, take a look at http://aafradio.org/ .  I spent a couple of hours looking at the photos. I still have somegear from that era around here, including a BC221 frequency meter (but without the calibration book), a Command Receiver for the 150 - 550 KHz band and a 1020 Hz "range filter."

 
 
13 August 2007

In addition to my ferrite rod article, I received an acceptance letter for an article on the Z100 CW tuning aid. It will also appear in QEX over the next few months.

I've finished the article on resistors and submitted the manuscript to QEX today. It's about 30 double spaced pages and 35 illustrations. It includes the material on my Carbon Composition Resistor page, but goes way beyond that, as I've provided extensive RF measurements over the range 300 KHz - 500 MHz for carbon composition, carbon film, metal film and thin film parts. The following illustrations may provide the flavor of the article.
 

 

In case anyone wonders, writing an article like this requires a great deal of work. The measurements are a fraction of the total effort, with most of the effort being in research and writing, drawing graphics and double-checking and  triple-checking the text. The resistor article, for example, required more than 120 hours. That's about four hours for each manuscript page.

 

08 August 2007

I will have another article in QEX concerning ferrite rods and their perversity, as I've recently received a letter accepting my manuscript. I use the  term "perversity" only semi humorously, as they have some of the most non-linear relationships I've run into. The inductance depends on about everything, including where the winding is with respect to the rod center, the percentage of rod length covered by windings, the ratio of the rod diameter to length and the rod permeability to just name the most important ones.

I hope to finish another manuscript this week, concerning resistors. A few weeks ago, I measured a stash of new old stock 1/2 watt carbon composition resistors I ran across in a distant corner of my garage, report here. Those measurements lead me to further explorations in the world of resistors, and I hope to finish a draft of the article and get it off to QEX by Monday.

 

04 August 2007

I've added a section to my Prototyping page showing Teflon push-in pins, and a simple home made insertion tool. These pins are available at modest cost on the surplus market and offer a useful solution to those portions of a circuit that can't be adequately built with conventional Manhattan-style construction. The section can be found on the Prototyping Page by clicking on the Teflon Pin Inserts link. Or, some browsers will take you there directly by clicking here.

 
 
02 August 2007

I've received a question about the compatibility of 50 and 75 ohm BNC connectors, as the Extron ADA 6 300MX video distribution amplifier is equipped with 75 ohm female BNC connectors. I've confirmed my understanding that 50 and 75 ohm BNC connectors are mechanically interchangeable. Amphenol's web site notes the following:

Two distinct types of 75 Ω BNC's are available, and both mate with each other and with 50 Ω BNC's. Type 1 is designated 75 Ω BNC-T1 and provides constant 75 Ω performance with low VSWR DC 4 GHz. Type 2 is designated 75 Ω BNC-T2 and is usable with low reflection DC - 1 GHz. For applications above 1 GHz, Type 1 is recommended.

http://www.amphenolrf.com/products/bnc.asp?N=0&sid=46B11E806D75617F& [emphasis added]

Interchangeability is definitely not the case with Type N connectors, as a male 50 ohm N connector has a larger pin diameter than a 75 N connector. Hence, mating a 50 ohm N male connector with a 75 ohm N female connector will damage the female connector's spring fingers. Likewise, a precision 75 ohm N male connector mated with a 50 ohm female N connector will not make reliable contact.

In the BNC case, however, the pin's mating end diameter is identical in both the 50 and 75 ohm versions. The impedance difference is accommodated by reducing the plastic insulation diameter, which raises the connector impedance by making part of the connector structure air. (Air has a lower dielectric constant than the plastics used in the connector. Since coaxial cable impedance is proportional to 1/square root (C), decreasing C increases the connector impedance.) There's also a difference in the pin shank diameter between 50 and 75 ohm BNCs, but the critical dimension is the mating section diameter, identical in both 50 and 75 ohm BNC connectors.

As a final check, I measured the tip diameter of the 75 ohm BNC connector on the video cable supplied with the ADA 6 300MX and compared it with the diameter of a 50 ohm BNC's pin. To my ability to measure, they have the same diameter.

And, the impedance mismatch from using a 75 ohm source with 50 ohm connectors and cable is unimportant when routing 10 MHz frequency data short distances around my basement lab.

I also looked at the waveform out of the ADA 6 300MX video DA when connected to my HP 5065A Rubidium frequency standard. The image below shows the waveform when connected to the oscilloscope's high impedance input. The peak-to-peak level is 4.6 V and the waveform generally looks good.

When terminated with 50 ohms, the waveform drops in amplitude

 

When terminated with 50 ohms, the waveform drops in amplitude, as expected.  Don't pay attention to the oscilloscope's frequency measurement in the image below. It's really 10.000000 MHz as shown in the first image.

 

 
01 August 2007

As usual, I've moved the July 2007 Updates page to archives. It may be reached by clicking here, or via the link table at the top of this page.

Most recent test equipment has an option for synchronizing its time base to an external standard. Some receivers also have this option, and Elecraft's K3 transceiver provides external synchronization as well.

I have an HP 5065A Rubidium frequency standard, but it has only a single output for each frequency (10 MHz, 5 MHz, 1 MHz and 100 KHz), so a distribution amplifier is normally used to connect multiple pieces of test equipment to a single time base standard. HP's 5087A distribution amplifier is commonly used, but can be difficult to find configured in the way that best meets your test bench. The 5087A uses a separate plug-in card for each output port, and the cards are frequency specific, although it isn't too hard to make a 5 MHz card into a 10 MHz card. Other distribution amplifiers are available, including a kit from TAPR, as well as commercial products.

A recent posting on the Agilent mailing list, however, suggested using an inexpensive video distribution amplifier, an Extron ADA 6 300MX, for example. This video DA has three independent channels (red, green and blue), each with 6 isolated outputs. (There's also a synchronization channel, but that is more difficult to use for this purpose.) This allows, for example, 6 outputs at 10 MHz, 6 outputs at 5 MHz and 6 outputs at 1 MHz. Or, the output of one channel may be connected to the input of another to give, e.g., 11 outputs at 10 MHz.

I found a number of ADA 6 300MXs on E-bay and purchased one for $49.95, which is about one-tenth of the price of a test equipment type time base distribution amplifier. It arrived today and I've made a few quick measurements and installed it in my basement shop. It seems to work well.

My current configuration is: 
Red input = 10 MHz, with 6 outputs
Green input = 5 MHz with 6 outputs
Blue currently unused.

The video DA is 75 ohms input/output, but for distributing 10 MHz and lower time base signals around my basement shop, this mis-match is not important. Furthermore, although the video DA is equipped with 75 ohm female BNC connectors, the connection end of 50  and 75 ohm BNC connectors is identical, so 50 ohm male BNC connectors may safely be used with these DAs. (There's a difference in the insulation diameter between 75 and 50 ohm connectors.)

I understand from the Agilent mailing list that it is possible to change the input/output impedance from 75 ohms to 50 ohms via resistor changes. Since this is not necessary, I have not further explored the subject.
 

Front view of the ADA 6 300MX.
 
Rear view. The unit is powered via a 120V mains connection.

The output BNCs are 75 ohm female type. The connecting portion of center pin's diameter is identical in 75 and 50 ohm BNC connectors, so no damage will result using 50 ohm male BNC connectors with 75 ohm female connectors..

 

These video amplifiers have excellent high frequency response, and are usable to nearly 400 MHz. The response will change depending on two option switch settings, AC/DC coupled and 1V/0.7V. The frequency sweep below shows the flattest response, obtained with AC coupling / 0.7V gain. Marker 1 at 1 MHz is set to be 0 dB relative, so marker 2 shows point at which the output is -3 dB with respect to a 1 MHz input.

I don't know what the noise figure of the amplifier is, so I don't necessarily recommend one as a DA for VHF signals, but this shows the amplifier to have more than enough frequency response for timing signal distribution.
 

 

The next plot sets Marker 1 to absolute mode, and marks the spot at which the next gain is 0 dB, 368 MHz. The peak gain is about 2.5 dB.