www.makarov.ca - A Digital Step 70dB 1GHz Attenuator

A Digital Step 70dB 1GHz Attenuator

A wide-band digital attenuator helps with your RF measurements

A small signal electrically controlled step attenuator is a desirable accessory for every RF designer or experimenter. In the last few years a few homebrewed or factory assembled Vector Network Analyzers were developed and then repeated or purchased by hundreds of Ham Radio enthusiasts. Frequency range covered by these devices extends from VLF through HF into the lower GHz range and the output power of such a VNA is often fixed in the range of +5 to -20dBm depending on the design. It is desirable in some case, however, to control the output power. An example is measuring the S-parameters of a small signal transistor with a VNA, or assessing linearity or compression point of a circuit.

The digital step attenuator offered in this article was designed to meet the needs of the owners of the homebrewed Test equipment and owners of the N2PK VNA in particular. The N2PK VNA itself covers a frequency range of 0.05...60MHz. However this digital attenuator works well over a much wider higher frequency range up to 1GHz and can be used with many other types of Test equipment such as other DIY VNAs, signal generators, spectrum analyzers, receivers an other equipment within the 1GHz frequency range.

The design for this attenuator is based on the RF switch ICs that connect discrete 10,20 and 40dB resistive attenuation sections to the main channel line. This allowed to maintain good Return Loss across the working frequency range and reduce ripple in the Transfer Function. In fact there is no ripple and attenuation slightly changes in a monotonic manner towards the upper frequency limit. Because of the monotonic behavior of the Transfer Function and Return Loss, the Group Delay function is also flat, which makes this attenuator suitable for use in applications where minimal variation in the Group across the working frequency range is desired.
All of the existing single-chip digital step attenuator ICs that had been evaluated at the design research stage were eliminated from consideration because they typically have poor Return Loss or have high ripple in the transfer function even within a short range of frequencies. Also the integrated attenuator ICs are often very small and have tiny packages that would preclude many builders from repeating the design, or require a communication interface such as I2C or SPI to control the attenuator which is rarely available. Also, the typical attenuation range of the integrated attenuators is less than 31dB and they have to be connected in series to increase attenuation.

This 70dB 1GHz attenuator is controlled via the three control lines by using the 3-bit binary code. A combination of the signals on the control interface activates a specific attenuation section or a combination of sections with a 10dB step. The insertion loss changes from 0 to 70dB with this step. The design of the board allows to use either SMA or UFL type of RF connectors as well as different connector mounting options. The board photos above this text and to the right of it shows the SMA connectors mounted on the same side and on the opposite sides of the PCB. The SMA connectors can also be mounted in a right angle manner on the adjacent sides of the PCB.

The attenuator has an optional 1-digit 7-segment LED display which indicates the current attenuation level represented by 1 digit from 0 to 7 as shown in the following photo. The PCB allows installation of either Green or Red LED display depending on the builder's preference.

The digits displayed are 0 to 7 and represent the current attenuation level. In the photo on the left the attenuator was set to 40dB attenuation. It is not necessary to solder a LED display on the PCB. If desired, the display can be mounted on the front panel and connected to the PCB via a wire header. This allows use of any 7-segment LED display with up to 5mA of current per segment, even the ones with a pin layouts different from the one the PCB was developed for.
A display is completely optional and was provisioned mainly for convenience. The RF part of the attenuator does not dependent on it.

Another connector option is a mini-coaxial UFL type of RF connector as shown in the following photo. UFL cable assemblies are available on eBay and many RF part suppliers. This setup allows the board to be mounted in a bigger box and the coaxial headers extended to the front panel and the bulkhead RF connectors mounted at the most convenient places on it.

The PCB layout was designed in a way that allowed to use wire or SMT jumpers to configure the RF connector options depending on which connector type the builder intends to use and where the builder wants to place the connectors. It is also possible to use SMA and UFL connectors together, e.g. use a SMA edge mount female connector on the attenuator input and use a UFL miniature connector with an extension RF cable at the attenuator output.

Specification

The 0,10,20 and 30dB step cells have flat attenuation up to 1500MHz. The 40,50,60, and 70dB step cells have 3dB cut-off bandwidth of 1000MHz. The Return Loss values specified in the table are the minimum ones and typically are better than 20dB. Another benefits of this Attenuator is extremely fast switching time of only 200ns according to the IC manufacturer datasheet. As long as the control lines change in sync, all of the attenuator cells will switch at the same time.

The harmonics level could not be measured because it was smaller than my HP spectrum analyzer's measuring limit of 70dBc. In reality it can be well better than that.

	Port Nominal Impedance:		50 Ohm
	Attenuation Values:		0 to 70dB with 10dB step
	Main Line Insertion Loss (0dB position):		3...4dB from 1 to 1000MHz
	Frequency range:		0 to 30dB: 0.5...1500MHz, 40 to 70dB: 1000MHz
	Bandwidth:		1dB cut-off: 500MHz, 3dB cut-off: 1000MHz
	Input Return Loss:		>17dB (VSWR <1.3)
	Output Return Loss:		>20dB (VSWR <1.22)
	Harmonic Level (Input Power 10dBm):		Better than 70dBc
	Switching Time:		200ns
	Maximum Input Power:		20dBm
	Control Interface Input:		Binary 3-bit with pull-up resistors to +5V rail
	Power Supply Input:		+5V @20mA
	Board Dimensions:		2x1-7/8'' (50x47mm)

Attenuation vs Frequency (normalized to 0dB cell):

Attenuation Cell, dB	10MHz	500MHz	1000MHz
10	10 dB	10 dB	10 dB
20	20 dB	20 dB	20 dB
30	30 dB	30 dB	30 dB
40	40 dB	39 dB	37 dB
50	50 dB	49 dB	47 dB
60	60 dB	59 dB	57 dB
70	70 dB	69 dB	67 dB

Operation

The control interface was designed for use with an open collector or open drain driving stage. The attenuator has built-in pull-up resistors tied to the positive power supply rail. The driving logic is positive, i.e. a logical "1" will activate a cell. When the control interface is disconnected, the attenuator will be on the maximum attenuation (70dB) because all 3 control lines will have a logical "1" supplied via the pull-up resistors. A 3.3...5V CMOS driving stage can also be used.

The Attenuator has to be powered on before the RF power is supplied. The used RF switch ICs are of the reflective type and present a very low resistance (low Return Loss) on the Attenuator input and output when powered off.

This attenuator can be used with the N2PK VNA. The recommended software is myVNA from G8KBB, which can be configured to control the attenuator. The attenuator connects to the N2PK VNA board v5.x through the connector J702 Att0..Att2 control lines. The v5.x board connection layout diagram can be downloaded from here. For connecting the attenuator to other N2PK VNA board versions please refer to the N2PK VNA documentation.

Testing Equipment

Development and testing of this little attenuator was performed using some serious equipment. The HP 8753C 6GHz Vector Network Analyzer was the leader on the bench. Without one it would be difficult to measure the attenuator Return Loss characteristics.

Because of working at HF to UHF and into the microwave region, beside using the appropriate test gear having quality RF supplies and tools was also a must. Lab quality industrial 50 Ohm loads made by HP and Minicircuits were used as the calibration standards. Semi-flexible low RF leakage coax jumpers were used for interconnections. The equipment on the bench included:

	HP 8753C Vector Network Analyzer 0.3MHz...6GHz
	HP 85047A S-Parameters Test Set 0.3MHz...6GHz
	HP 8565A Spectrum analyzer 0.01...22GHz
	HP 8444A Tracking Generator 0.5...1500MHz
	HP 5334A Frequency counter
	HP 3456A Digital Voltmeter
	HP 8656B Signal Generator 0.01...990MHz
	Tektronix 2467B 400MHz Oscilloscope
	Tektronix 2430 Digital Storage Oscilloscope
	Lucent RFG-M-RB Rubidium Frequency Reference Standard

Most of the equipment was cross-checked and the results agreed to each other. Based on that it was then assumed the equipment was performing up to its specification.

Performance verification

A series of tests was performed at each of the attenuation steps using the HP 8753C network analyzer and the resulting plots merged in one in a graphics editor to produce a reconciled attenuation plot which is shown in the following picture. The analyzer was set to display attenuation relative to the "0dB" cell.


It can be seen that the 10/20/30dB step cells exellently maintain their attenuation levels thru to 1GHz and the traces are not even visible on the plot because of being masked by the grid. Same is true for the 40/50/60/70dB step cells which begin to roll-off only above 300MHz.

PCB and parts kits availability

Please check this link for availability of the Attenuator kits. A kit includes a PCB and a set of 6 RF switch ICs. The rest of parts can be obtained from DigiKey using the BOM document available in the Downloads section of this article. Cost of a DigiKey parts kit is approximately $10 not including the RF connectors. It is left to the builder to select a connector option based on the local needs. In a minimalistic setup the coaxial extension cables can be soldered directly to the PCB.