AX5045 Component Determination

Hello,

I am designing a transceiver circuit using AX5045. The datasheet, pages 31 and 32, shows how to determine the component values. I started to calculate component values using the AX5045 DEV board (ADD5045-915-1-GEVK). However, except for the 3 component values, the design is not matching with my calculations. I just want to get more information about this design. Unfortunately, the datasheet is not providing deep information about design-wise. I am looking for any helpful documentation or any supportive comments.

Thanks,

@ofgms

Please refer to the external component values of our ADD5045-915-1-GEVK: AX5045 915 MHz Add-on modules for DVK-2 Evaluation Kit and schematic + BOM for 915Mhz settings.

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Hello. In addition to the reply above.
Please note that the ADD5045 uses the match for "Direct RX/TX Connection (Figure 9)" which is derived from the "Separate RX/TX Connections (Figure 8)". As reported on page 34:

A co−match for the PA and LNA is possible with some compromise to performance as shown in Figure 9. A large coupling capacitor Ccm is used to connect them together. The initial values of the PA and LNA input matching components are calculated using the previous equations.

So the equations in the AX5045 datasheet on pages 31/32 are provided for the "Separate RX/TX" case.

Hi @rmozel,

  1. To re-iterate design calculations for 915MHz EVK using equations given in data sheet p.31, one needs to know selected of values of some design parameters (and/or related register settings). The design equations given in data sheet dominantly depends on Q_{L}, V_{choke}, V_{sat}, and P_{out}. What are the selected values of these parameters for 915MHz EVK?

For the calculations we used the following typical values: Q_{L} = 2.5; V_{choke} = 2.8V; V_{sat} = 0.7; P_{out} = 0.1.

  1. According to the DS, the value of V_{sat} is 0.7V approx. but it states that it may be adjustable! Is there any programming mechanism to set V_{sat}? The same question is also valid for the nominal value of V_{choke}. Is nominal voltage of V_{choke} 2.8V or it is adjustable with max. 2.8V? Should we take it as a constant in calculations? If not, is there a way to set V_{choke}?

V_{sat} is to account for the saturation voltage of the switch transistor of the PA. This is not a programmable value (on the chip), but it can be adjusted slightly in the equation to achieve better correlation between the measured and expected power. V_{choke} should typically be 2.8 V in the equations as that is the max voltage available on the V_{choke} pin. Using the max voltage will result in the maximum efficiency. However, note that the V_{choke} voltage varies as the TXPWRCOEFF registers is changed, as this register controls the output power.

  1. For setting transmitter output power, there is Table 19. of AX5045 Programming Manual but one can not find any details about TXPWRCOEFF{A-E}. Is there any relation between P_{out} and V_{choke} (and possibly V_{sat})?

The relationship of V_{choke} and P_{out} is as you would expect. Where a change in P_{out} is related to a change in V_{choke} ^2. The TXPWRCOEFFA/B/C/D/E are controlling the predistortion performed on the transmit signal as shown by the highlighted function from the programming manual:


where a_0=TXPWRCOEFFA, a_1=TXPWRCOEFFB, a_2=TXPWRCOEFFC, a_3=TXPWRCOEFFD, a_4=TXPWRCOEFFE.

Thank you for the note on the wrong equation, it is indeed a typo.

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Hi @rmozel, what wat the P_{out} you used in your calculations? As mentioned before, we have designed our DVK to operate at max power at the antenna of 23 dBm (200mW). As the circuit is differential, the P_{out} per single branch is 100 mW.
In addition, note that the equations just gave a starting BOM, but the final components choice was done experimentally and iteratively on the actual PCB.

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