The GPSDO; what is it and why do I have one

I purchased this GPS Disciplined Oscillator a few weeks ago. The reason being, I wanted to make sure that this frequency counter was accurate.

Hewlett Packard 5315A Universal Counter

This Hewlett Packard 5315A was last calibrated in 1990. That made me suspicious. While I could send it back to Agilent and have it recalibrated, I thought it might be interesting to check it with a known standard.

When I connected the frequency counter to the 10 MHz GPSDO, it was -2.1 Hz off. At first, I thought perhaps the GPSDO was off; however, the spec for the LBE-1420 is 1 x 10-12 with a resolution of 1 Micro Hz. I let the HP unit warm up for 3 hours thinking maybe it was cold and would come back in tolerance. Nope, the frequency stayed about 2 Hz low.

It took about five seconds to find the full service manual online, which gives the alignment and calibration procedure in detail.

The first step is to use a DVM and check the +3, +5, and -5.2 power supplies. If they are off then adjust each accordingly. The next step is to check the +5 VDC pin on the Option 4 OCXO module and adjust as needed.

‘scope lead connected to reference oscillator pin

The calibration procedure for the HP 5315A is to connect a known 10 MHz reference to one channel of an oscilloscope and the output of the frequency counter OCXO to the other channel and look for slippage of the two signals. If the counter is on frequency, there should be no movement between the two waveforms. This is more accurate than trying to adjust the counter while looking at the frequency display on the counter.

Frequency alignment HP 5315A, Yellow squarewave trace LBE-1420 GPSDO, Violet sinewave trace 5315A reference oscillator

When I first connected it, the HP’s waveform was running backward at a pretty good clip. I adjusted the OCXO until there was no movement relative to the two waveforms. I let it sit like this for about three more hours before buttoning the HP unit back up. I am confident that the frequency counter is accurate +/- 0.3 Hz, which is good enough for my purposes.

LBE-1420 GUI

What I like about this Leo Bodnar GPSDO is that you can change the output frequency to any value between 1 Hz and 1.4 GHz. The output level is +13 dBm (per data sheet) with low phase noise, making it an excellent portable signal generator. The output is a squarewave, however, installing an LC type bandpass filter such as a Mini-Circuits SBP 10.7+ will round that out into a nice sinewave.

The Leo Bodnar website has a portable Windows executable for download, which can be used to program the output frequency and monitor performance.

I measured the output with my precision power meter; at 10 MHz it was +10.35 dBm. The low power output setting is about +5 dBm.

WWV carrier measurement with LBE-1420 as external 10 MHz reference

Another use for the LBE-1420 is as an external 10 MHz reference for test equipment. My Network Analyzer (and many other pieces of test gear) has an external 10 MHz input and if I use the spectrum analyzer to measure carrier or pilot frequency, it is nice to know that the test equipment is exactly on. I confirmed this by measuring the WWV carrier with my Siglent SVA-1032X spectrum analyzer using a long wire antenna.

Inexpensive Chinese GPSDO

Continuing with this interesting topic, I purchased a fairly cheap version from Ebay for further research. This particular unit is a clone of a BG7TBL, which is itself a clone. The interesting thing about these units is that they are using recycled OCXOs, which appear to be from decommissioned telecom equipment.

BG7TBL GPSDO block diagram

This diagram shows how these units work. The GPS signal is received by the GPS module, in this case, a uBlox M-7. The NMEA sentences and 1PPS are fed into the CPLD (Complex Programmable Logic Device). The NMEA sentences are also available on the RS-232 DB-9 connector.

GPSDO component side, GPS module lower left

The CPLD takes the output of the OCXO, in this case, a CTI OC`12SC38A, and compares the 1PPS from the GPS module to the 10 MHz from the OCXO module and adjusts the OCXO module by varying the voltage on the frequency adjust pin to keep it on 10 MHZ. It then sends the corrected 10 MHz and 1PPS signal out to BNC jacks. I found the 10 Mhz output level was +13.58 dBm as measured with my precison power meter. There is a built in bandpass filter, so the output is a good looking sinewave.

Judging by the CTI model number, it was made before 2015. There should be a date code on the bottom of the unit, but I did not feel like unsoldering it.

GPSDO OCXO

The one issue with this; OCXOs frequency drifts over time and eventually it will be out of the adjustment range. A closer look at the circuit board shows that it will accept several different OCXO modules. These modules run about $40-60 US new and $10-15 US used.

If an OCXO is suspected of being out of adjustment, they can be measured using the osciliscope method noted above.

Installing a 60 KW FM transmitter

Recently, I installed this very nice GatesAir FAX60HD.

This project was for WPGC, Washington, DC. WPGC (Prince George’s County) is an Audacy station with a Hip-Hop and R&B format. I was listening to The Good Morning Show on my drive from the hotel to the transmitter site, and those guys were hilarious! It’s nice to hear a well-programmed radio station.

It is always fun to accept new and interesting challenges. This is, to date, the largest transmitter either AM or FM that I have ever installed. Previously, I installed several FLX-40 units, which is quite a bit of power for the FM side of things.

MSC unit with touch screen pad controls both transmitters and exciters

This transmitter combines two FAX30 transmitters and is controlled by an MSC unit. The content stream for HD comes from an FMXi4g, which has several great features.

BDI inline watt meter

This station’s TPO is 45.7 KW with the HD carriers at -14dBc. While this is a class B station with an ERP of 50 KW, the four-bay half wave-spaced antenna requires a lot of wattage to make that TPO. This is a largely residential neighborhood, which is, I surmise, the reason for the half wave-spaced antenna.

WPGC main and backup antennas, Capitol Heights, Maryland

I was told that this is not the greatest part of town. The station has had some theft of outdoor air conditioner equipment in the recent past. That being said, it is much nicer than many areas we normally work in the NYC metro area. The transmitter site has been here since the station signed on in 1948.

WPGC transmitter site
FAX60HD power supplies and power amps installed

It took a bit of time to install the 42 power supplies and 48 power amps. The power amps were installed in the same slots as during the factory test cycle. Thus the data on the test sheet matches the data seen on the transmitter GUI when we turn it on.

FAX60HD cabinet interconnects completed

All of the cabinet interconnects; RF plumbing, grounding, AC supply, sample lines, and various control lines were completed.

Transmitter hybrid combiner for the two FAX30 transmitters
FAX60HD, WPGC-FM Washington
4 Inch Dielectric coax switch with 60 KW load
WPGC 4th harmonic

Most of the harmonics (2-10) looked like this. However…

WPGC 5th harmonic

This is something interesting that came up during the proof. When measuring the harmonics, most of them were in the -130 dB range. This one is slightly higher than that, which is due to the proximity of WFDC-DT on channel 15 (476-482 MHz), 1000 KW ERP about 10.6 miles away. Their signal was coming back down the transmission line from the FM antenna. This is a good demonstration of how other unwanted signals can get into the final sections of transmitters which can cause intermodulation mixing products. In this case, the FAX60 has several low-pass filters that remove this and other signals before that happens.

This is replacing a pair of combined BE FM25-T transmitters that were getting a little bit long in the tooth. The air staff has commented on the noticeable improvement of the station’s sound. The downside of tube transmitters is the delicate tuning procedure to reduce the AM noise. High-powered transmitter tubes are also getting more expensive and, for some types, harder to source.

For Sale: WZLH-LD

Does anybody want to buy an LPTV station?

For several months now, I have been taking care of an LPTV station in Syracuse and the owner is looking to sell. It is in a pretty good location; on a tall building downtown. It has a newer transmitter and antenna, EAS gear, located in a conditioned space, etc.

State Tower Building viewed from Montgomery and Water Street. Syracuse, New York

If you are interested, contact me for details: info (at) engineeringradio (dot) us

More AM work, Part V

I don’t know how many parts there should be in this series, five is a guess.

While some AM stations surrender their licenses to the FCC, others are undergoing needed repair work to stay on the air for a while longer.

WBEC-AM, Pittsfield, MA

WBEC-AM in Pittsfield Mass is the topic of today’s post. This station is Non-Directional day, Directional Night with a two-tower array. This site was built around 1956 when the station moved from Eagle Street, near downtown Pittsfield.

WBEC-FM backup antenna, mounted on WBEC-AM array

This Shively 4-bay antenna is the backup antenna for WBEC-FM. It is being replaced with a 3-bay antenna. The new antenna will serve W277CJ which is relocating from downtown and as a backup for WBEC-FM. A set of signal strength measurements for the nighttime directional array is required along each of the three monitor point radials before and after that work is completed.

The issue these days is the nighttime directional system, which is somewhat erratic when in use. The towers are 180 degrees tall with 35 degrees of top loading making the towers 215 degrees tall or just under 5/8 wavelength. As such both current and voltage are near maximum at the tower base, which makes them very sensitive to any changes at the base.

First, there was an issue with the tower lighting system. It seems that somewhere on the non-reference tower, the beacon conductor is shorted to the tower. When the tower lights are on, the loop current rises and falls in time with the beacon light. Because these are series excited towers, the fuse for the beacon does not blow, but the 60-cycle AC current does show up on the tower loop current reading. Rather than try to repair things on an almost 70-year-old tower light system, it was decided that both towers should get new LED lighting systems.

Next, mice chewed through several AC supply cables for the phasor at the base of tower #1. When switching from day to night patterns, some or all of the contactors would not move or get hung up between states taking the station off the air.

It is unfortunate that the phasor is at the base of one of the towers and the antenna monitor is back inside the building with the transmitter. It takes two people to make adjustments to the nighttime array.

The tower #1 ATU/phasor building is full of old unused equipment, a mouse haven. It also blocks access to several points that need to be sealed up to keep mice from getting in in the first place.

Slatercom LED light system controller

What is interesting is, that the original wiring and photocell failed at some point, and someone simply wired up a new photocell, leaving all of the old equipment in place. Thus, it became difficult to troubleshoot and ID the conductors in use vs the ones abandoned. With the replacement of the tower light systems, I decided that everything must go. The new Slatercom A-1 replacement lighting systems have individual controllers each with its own photocell. In addition, they have wireless links for tower light monitoring. It is great to get rid of the dry pairs going back to the transmitter room, which always creates RF and lightning headaches.

The station will also be saving some money on electricity. The new lighting system draws 88 watts total vs the old incandescent system which drew 1,000 watts with a 50% duty cycle on the beacons. The old system was on all the time due to photocell failure. I estimate they used 17,520 KWh per year on tower lights, at $0.20 per KWh which is $3,504.00 per year vs $308.00 per year for the LED systems. The added benefit of LED fixtures is that they should last much, much longer than incandescent fixtures.

WBEC-FM Harris FM1H3 transmitter scrapped

This lovely Harris FM1H3 was donated to the scrap yard. I believe that this is the second transmitter (1974) for the original 105.3 WBEC-FM which signed on in 1967.

Tower #1 ATU building cleaned out

The building clean-out, unwiring, and rewiring process took about a day and was well spent, in my opinion. Working in a building that is not full of mouse nests, droppings, and stinks like mouse urine is nice. I plugged several holes in the building with stainless steel pot scrubbers and spray foam.

Nautel Amphet 1 transmitter

After removing and replacing the old tower lighting system, the Nautel Amphet 1 transmitter would not run into the nighttime array. This was likely due to the changes at the tower base. I used a VNA to measure each tower’s base impedance which is 42.5 ohms and -j139, 44.5 ohms -j155 respectively.

WBEC Tower #1 base impedance
WBEC Tower #2 Base Impedance

Then the daytime and nighttime common point impedance was measured. Both were off, but the nighttime was more so than the daytime. I adjusted the R and X until both were reasonable and the transmitter would run into both patterns. According to Nautel, the Amphet 1 transmitter runs best into a load of 50 ohms j+5 measured at the transmitter output.

WBEC antenna system schematic

The daytime antenna is non-directional and there is no “common point,” however, there is an R and X adjustment in the phasor for the daytime tower. According to the file I found, it used to be a directional daytime until 1967, when they could make the daytime antenna non-directional. It is an interesting setup.

With the array properly adjusted, readings could be made along the monitor point radials. This station has three monitor points, two are in the nulls and one is in the main lobe. It is the nulls that are the greatest concern. Fortunately, much of the documentation from the original proofs was found in a filing cabinet. While the maps are nice, they date from 1950 and are woefully out of date. However, I was able to find good reference points on the 1950 maps and redraw radials on a Google map.

I found these photos of the monitor points as they were in 1956:

1956 photographs of an engineer making field strength measurements

Those were great because I was able to verify the locations of the monitor points today:

243-degree radial monitor point, 2024

Based on that, I made three maps with radials on the monitor point azimuths:

WBEC-AM Pittsfield, MA 243-degree radial ten measurements

The consulting engineer wanted about 10 readings on each radial. I created an individual map for each radial, marking points where the radial crosses a public road. When it came time to do the monitor points, I loaded the map into my smartphone and followed the directions to each point. It worked very well.

We will return and make post-installation measurements once the new FM antenna is installed.